Your question captured my imagination and apologies for going off somewhat at a tangent in my previous reply. I live in Wiltshire and today I went to the Wiltshire Heritage Museum in Devizes where I spent some time in the Neolithic and Bronze Age galleries.

This is what I learnt today (prompted by your post):

The Chambered Long Barrows are from the Neolithic period and there is a high concentration of them around Avebury - 14 were pinmarked on the map. The most famous is West Kennet Long Barrow which was built 3600BC and sealed up in 2000BC so was in use for over 1000 years.

The longbarrows fall into two categories - Passage Graves and Gallery Graves. I could not find any information that made reference to your question, although the Gallery Graves have a large rectangular chamber set at the broader end of a long mound.

However, there was an old photo of the Lanhill Chambered Long Barrow, near Chippenham, and which has now been almost destroyed. The information said it was 56 metres long and had at least 3 chambers in the mound. It had a forecourt and probably a false entrance - so perhaps that's a clue.

On the way out I noticed a book for sale called The Megalithic Chambered Tombs of the Cotswold-Severn Region by TC Davill (published by Vorda) which looked interesting.