I actually (for a change) found this site quite easily.
There is nowhere 'proper' to park but there is room to just about pull into at the field entrance. Look over to the right and you will see a small copse of trees a little way into the field. This is where the remains of the Dolmen lay.
Getting into the copse is another matter! Completely surrounded by bushes and chest high nettles. I walked right the way around the copse and luckily spotted some large stones through the nettles/bushes on the eastern side. I carefully made my way towards the stones by trampling the nettles down one at a time (I was wearing shorts!).
Eventually I got to the several large, moss covered, stones. On the left was a stone which looked as though it could have been the cap stone - a large old tree had fallen across the top of it. Access further into the copse would have been very difficult so I didn't even try.
Well worth a look when in the area.
Tip - either bring some shears or don't wear shorts!
The EH Monuments record describes the site thus:
"The site lies on a gentle south east facing slope at the north east corner of a small wood.
The portal dolmen has one large upright and one adjacent inclined stone, together with a number of smaller stones on the northern side of a roughly square depression which measures 3m across and 0.2m deep. The upright limestone block measures 1.54m long, 0.72m thick and stands 0.94m high above the present ground level. The inclined stone immediately to the east measures 1m long, c.1m wide and 0.5m thick. Surrounding the central depression is a circular bank of small stones which measures c.10m in overall diameter. The bank is 2m wide and stands 0.4m high to the south."
I could see two possible candidates for the 'small wood', but couldn't find a way through the hedge from the road (which is a bit of a racetrack!) I'll have to leave this for someone more local to investigate further.
There is also a possible Long Barrow in the same vicinity, which is close to Chastleton Barrow Fort and the Goose Stones, so lots of evidence for this being an important centre at one time.
Some added information about the Burnt Hill Portal Dolmen (gleaned from the record on Magic):
Only about 20 portal dolmens are known in the country, so this is a rare (if unprepossessing) site. Most are in west Penwith, Cornwall, or in the north-west Oxfordshire Cotswolds (there are a few in between). They date from the Early and Middle Neolithic (about 3500-2600 BC) so are practically the oldest monuments you can find in the landscape.
When shiny and brand new the dolmen would have been a small closed rectangular chamber built from large stone slabs, with free-standing stones flanking the front slab. A (usually massive) capstone would have covered the chamber. It may have been reused centuries later for urned cremations in the Bronze Age.
Today the dolmen lies on a bend in the modern (and presumably ancient) county boundary between Warwickshire and Oxfordshire. Even in 1971 field observations turned up "two small fragments of human skull, two fragments of long bones, three struck flints, including a retouched flake, and two fragments of pottery." It does make you think about what will be left of our society's artifacts in 5500 years time.