I lived nearby in the village for three weeks in January 1993 (house and cat-sitting). Hiked up the hill to the clumps every day, and walked for miles through the woods and fields of winter barley. This is an extraordinary area! Even on damp wet days. That year January saw roses in the Abbey and forsythia in gardens; it had been a wet autumn apparently. I took a lot of photographs (35 mm) and I will try to scan them in and post them at some point...
If you see a raven when you visit the Clumps, keep your eye on it. It's said to be the guardian of a huge treasure, at a place called 'The Money Pit'. But I expect it'll be too wiley to give away the exact spot.
The poem tree that Riotgibbon mentions is a beech (the beech plantation clumps were created in the 1740s, by the Dunch family – hence the rather disrespectful name 'Mother Dunch's Buttocks'). As young vandals everywhere will recognise, beeches have ideally smooth bark for carving graffiti into, and it persists for years, becoming more and more distorted as the tree grows. Apparently the tree died in the 1990s but some of the trunk still stands. In 1994 a plaque was put up to commemorate the 150th anniversary of the poem, and it has on it a tracing of the poem that was taken in 1965, when it was a bit more legible. (see http://www.northmoortrust.co.uk/home/countryside/nature_reserve/past_land_use for more)
Joseph Tubbs, 1844 - carved this poem on "Poem Tree" up the Clumps ...
As up the hill with labr'ing steps we tread
Where the twin Clumps their sheltering branches spread
The summit gain'd, at ease reclining lay
and all around the wide spread scene survey
Point out each object and instructive tell
The various changes that the land befel.
Where the low bank the country wide surrounds
That ancient earthwork form'd old Murcia's bounds.
In misty distance see the barrow heave,
There lies forgotten lonely Culchelm's grave.
Around this hill the ruthless Danes intrenched,
and these fair plains with gory slaughter drench'd,
While at our feet where stands that stately tower
In days gone by uprose the Roman power
And yonder, there where Thames smooth waters glide
In later days appeared monastic pride.
Within that field where lies the grazing herd
Huge walls were found, some coffins disinter'd
Such is the course of time, the wreck which fate
And awful doom award the earthly great."
The artist Paul Nash painted Wittenham Clumps many times - they had great symbolic significance for him. There is another painting of the clumps on this page: http://www.bhikku.net/archives/03/jan03.html (scroll down to see 'Landscape of the Summer Solstice').
You may find his work interesting and well worth exploring as he visited and painted many of our ancient landscapes.