IF you think of the Black Mountains as an upturned hand, the Olchon Valley lies in a gap between the thumb and forefinger.

It is in this Herefordshire cleft between the severe slopes of Black Hill and Hatterall Ridge that Owen Sheers sets his 2007 book Resistance.

In it, the 1944 D-Day landings have been unsuccessful and Britain is under Nazi occupation. A farmer’s wife wakes one morning at Upper Blaen to find that her husband has disappeared along with all the men in the valley.

A German patrol arrives and installs itself at Olchon Court. In a harsh winter, Sarah Lewis begins an acquaintance with Albrecht Wolfram. Cut off from the surrounding war, the lines between collaboration, duty, occupation and survival become blurred.

Our walk starts from a commanding position on the wives’ side of the valley.

We pass Chapel Farm, the site of a very early Baptist chapel well away from persecutors. (“Olchon” is Welsh for washing, baptising). Ruined farmsteads lie directly below the arched back of Black Hill: the source of a portentous bullet in the book.

The wind, if not Albrecht’s scratchy gramophone playing Bach’s Cello Suite No 4, now whistles through the rafters of Upper Blaen (point two on our route) at the head of the valley.

The isolated upper Olchon was familiar to a real historical figure.

For Henry IV against Owain Glyndwr, Sir John Oldcastle helped in the capture of 300 Welsh rebels at Usk and was a joint leader in the siege of Aberystwyth.

Later a trusted aide of Henry V in Burgundy, he became associated with religious radicalism and was tainted with unlicensed Baptist preaching.

He was convicted of heresy and captured in 1413, but escaped with a price of 1,000 marks on his head.

“Olchon Court sat further down the valley’s western wall. It was a square, solid building, a 14th-century fortified farmhouse with walls several feet thick and initials carved into its pitted beams by boys over 700 years dead,” Sheers wrote in Resistance.

Nobody has ever actually mentioned seeing Oldcastle’s initials carved into the beams, but the tradition goes that he hid here by the rivulet where he was baptised.

In his days as a fugitive, Olchon Court (point three below) was the home of sympathiser Sir Walter Brute, a fellow “Lollard”. Oldcastle is said to have jumped from an upstairs window to get away from his pursuers.

In the same manner the farmers in Resistance disappear over Hatterall Ridge. On the other side is the Vale of Ewyas, the outside world and Llanthony Agricultural Show and Country Fair.

On our hillside route below the ridge a young nobleman’s burial chamber dating back to nearly 2000BC was discovered in 1932.

According to archaeologists, he was short for a “Beaker”, but very brainy.

In an altogether more lofty position another hiding-place can be perceived further down the valley.

In Resistance the coveted Mappa Mundi is secreted in a narrow crevice in the rocks of Red Darren, a sandstone cliff jutting from the head of a scree slope high up the valley’s wall.

The real safe-house in wartime for the treasure was Hampton Park and a Bradford-on-Avon coal-mine.

For the fictional Resistance wives the future is full of foreboding. The usual fate for collaborators is assassination and for resistance fighters a summary execution on the village green.

The reality for an Oxfordshire chaplain found guilty of sheltering Oldcastle was execution.

While the demise of Sir Walter Brute has been sketchily linked to Bodenham, Oldcastle was caught after four years on the run.

His own story ended with a particularly barbarous execution. He was suspended from a gallows by an iron chain, a fire was kindled beneath him, and he was slowly burned to death.

THE ROUTE Map: OL 13 Brecon Beacons East.

1. Black Hill picnic site (GR 287 327). Car Park with fine views, signed, and 3½ miles NNW of Longtown, through Llanveynoe.

Turn right up surfaced approach road to park beneath Cat’s Back. With your back to the information board, TR and walk back down to road you came in on. TR past Chapel Farm (early Baptist beginnings) and follow quiet lane one mile past ruined farmsteads (wives’ homes in Resistance) to left turn in road. (Ignore bridle path ahead to Black Hill). TL down road across Olchon Brook by bridge or ford. Rise few paces to where the lane turns left out of “V”. TR along muddy footpath through gate to ruined cottage.

2. Upper Blaen (home of “Sarah Lewis”). Return to road, and follow it ahead, as if you had turned left in the first place, passing The Firs and go just under 1 mile to Olchon Court (on left).

3. Olchon Court (base seconded by German patrol in Resistance, and John Oldcastle’s place of refuge?).

TR off road across stile, with trees left and stone wall right.

Sweep L through gateway, past marker post by stream. (Site of Early Bronze Age burial. Red Darren cliff is now just under two miles ahead of you – Mappa Mundi. The Hatterall Ridge above right). Cross stile, go left over ford, through gateway and follow L fence ahead through old copse. Cross stile, beyond marker post (wave to May Hill), cross ford and go few paces up to R and cross next stile. TL and follow sunken lane through kissing-gate down to lane. TL ahead down to original lane and TL for 50 metres.

4. Stile. TR off lane again over stile into pasture. (Views open up right to the Lower Olchon Valley). Follow marker posts down to bottom of valley. TR, cross ford, then f/bridge over Olchon Brook. Bear R up past waymarked t/pole. TR at second marker post, ford, beyond post and stile next to it. Go left, to right of cottage and out of garden via gate. TL up meadow (R of trees). Dodge back down L into sunken lane and cross stile in L corner. Bear L ahead up through silver birches, beyond marker post to unfenced road. TL, then TR steeply back up to car park.