SEVERAL years ago, I found myself with some pilgrims in the heart of Umbria. In the little town of Monterchi, Piero della Francesca's great painting of the Madonna del Parto is displayed in a former primary-school house that has been made to resemble a wayside chapel.

Noon struck as we stood in awed contemplation in the darkened room, a little huddle of foreign visitors, and spontaneously the words of the Angelus were taken up with quiet prayerfulness.

Visiting Somerset House on Armistice Day, I was standing in front of Stanley Spencer's Resurrection of the Soldiers as silence broke out in the gallery at 11 o'clock. I spoke the words quietly of Laurence Binyon's ode, the promissory refrain taken up by those around. We will remember them.

The tear-stained face of the woman next to me, wordlessly thanking me as we went our separate ways, may have spoken of a lost father or an aunt. Or was she grieving a more recent casualty of war? Since the Second World War, 1968 is the only year in which apparently no service personnel have died; the service of the Crown remains a higher calling.

All societies struggle to find ways to live and words for those who have died. Henry VI founded the College of All Souls of the Faithful Departed at Oxford (20 May 1438) in part as a national memorial to the Fallen of Agincourt and of the French wars. Across Europe, the 1920s brought forth an efflorescence in the building of memorials and markers for those who had died in war.

Somerset House and the Pallant House Gallery in Chichester have come together with the National Trust to mark this year's centenary with an exhibition of one such chapel's decoration.

Stanley Spencer and his Australian painter-friend Henry Lamb (1883-1960) had both served on the "forgotten" Front of Salonika in the Great War. In 1923, the troubled Spencer was lodging with Lamb in Poole.

Also in Macedonia had been a young lieutenant, Henry Willoughby Sandham, who later died of an illness that he contracted there. Sandham's sister, Mary Behrend, was a high-church Anglican who had married into a Jewish family. She and her husband, John, already owned a work of Spencer's when they visited Lamb in 1923, and were shown a series of drawings that Spencer was beginning to work up, based on his earlier experience of the Greek front line and also of being a hospital orderly in the Beaufort Hospital, Bristol.

The Behrends had planned a memorial chapel in the grounds of their home in Burghclere in Hampshire, and invited the 30-year- old Spencer to decorate it with his highly autobiographical work.

At the Slade, he had been part of the group of Neo-Primitives, alongside the likes of Gertler, Nevinson, and Bomberg, who all looked to the early Italians as a model. When he was considering enlisting, he had even written to Lamb, adamant that "If I go to war I go on condition I can have Giotto, the Basilica of Assisi book, Fra Angelico in one pocket and Masaccio, Masolino and Giorgione in the other." He tookhis favoured pocketbooks withhim; that of Giotto published by Gowans & Gray in 1909 is in the exhibition.

At the end of the hostilities, he vouchsafed to his sister that he would learn the technique of fresco painting for a church that he hoped to build with a college friend from the Slade. Therefore, when Spencer and the Behrends met at Lamb's house, his intention and their plans neatly dovetailed.

The publicity of the National Trust surrounding this touring exhibition makes much of bringing "the UK's 'Sistine Chapel' to London". How much Spencer would have hated this gross misunderstanding of his artistic intention.

Michelangelo and the painter from Cookham are poles apart, and although there are in his oeuvre hints of Luca Signorelli, who might have served as a common model, it is to Giotto that Spencer returned again and again.

As Simon Martin in a brilliant catalogue essay shows, the 1305 fresco on the end wall, depicting the Last Judgement, is the decisive inspiration of Spencer's 1921 Unveiling a War Memorial at Cookham, in a private collection. The chapel paintings at Burgchlere are a natural development from the same source.

Rather, the decorative scheme for the private chapel at Sandham, designed by Lionel Pearson and dedicated by the Bishop of Guildford as an oratory on Lady Day 1927, is Spencer's response to the great Scrovegni Chapel in Padua, which was built by a convicted usurer and painted by Giotto di Bondone (1267-1337).

In fact, Spencer never did learn fresco painting, and the side wall panels were painted on detachable canvas, and it is these that form the core of the exhibition. The north- and south-side spandrels, and the east-end wall of the chapel, shown in a projected image, are also painted on canvas, but have been glued to the fabric of the chapel and cannot be moved.

Spencer's experience of the years of conflict had been as an orderly in a hospital and in the trenches. At the very end of the period, he had also ended up in hospital; so the day-to-day paintings that he undertook at Burghclere from 1927 to 1932 drew on these three very different aspects of his life.

He arranged, in a carefully contrived sequence, the focus of the chapel to be on the central act of resurrection hope shown in the sacrifice of the altar. The north-wall arcades depict the day-to-day life before conflict and death; the ablutions of the soldiers, shifting kit-bags or sorting the laundry, offer a humdrum view of the world that we can know.

The chores in a hospital or in the barracks are as much a celebration of life as they are of the proximity of death, so that the menial becomes quasi-miraculous. Mischievously, Spencer once claimed that the cycle was "a symphony of rashers of bacon" with "tea-making obligato" (sic) as it appeared to those from the Front.

Then comes a change of gear. Dug-out (or Stand-to) is a tense military scene of operations at the Front beneath the spandrel that shows The Camp at Karasuli. Lamb and Spencer had no experience of the trenches of the Somme or of the muddied landscapes of France and Belgium; so there is a very different colour palette.

The white stone of the vertiginous rocky terrain of Macedonia is itself an extraordinary backdrop, much as the glare of a Palestinian day illuminates Lamb's spectacular piece Irish Troops in the Judean Hills Surprised by a Turkish Bombardment (1919), with which the exhibition opens.

Death and the inevitable finality of conflict are then transformed in the great east wall, where so many soldiers rise to new life, one still trapped in serpentine-like barbed wire, like a lost soul strangled by a snake in a Byzantine painting of Last Judgement.

Spencer intended that his east-end painting would merge into the holy table beneath it, for which a curiously wrought white altar frontal has the Johannine text "I am the Resurrection and the Life" at cross purposes with one from The Tempest, "We are such stuff as dreams are made on". Evidently, the first Bishop of Guildford raised no objections.

The artist insisted that the altar cross would merge into the crosses he had painted above it; so it is a pity that the exhibition's organisers have chosen to project an image of the undecorated altar, as this loses the point of Spencer's multiple crosses that surround the fallen mules and detritus of a world above, which is waking to a new dawn.

In October 1932, the artist expressed his belief in his achievement: "I think that the arched & predella pictures arranged . . . round a gallery would be impressive. . . they would blow the 'Gallery' atmosphere to the four corners of the heavens." They have now come to London, too late to give him the job that he thought they might land him, but a powerful testimony to one man's response to the vengeance of Mars and the reconciliation of Venus.

"Stanley Spencer: Heaven in a Hell of War" is at Somerset House, The Strand, London WC1, until 26 January (free admission). Phone 020 7845 4600. www.somersethouse.org.uk. It will be at Pallant House Gallery, 9 North Pallant, Chichester, from 15 February to 15 June (admission charge). Phone 01243 774557. pallant.org.uk