I would guess you are looking for something around the 40 litre volume mark for a intermediate size.

One of the packs from the Karrimor Sabre SF range would most likely fullfill your requirements

http://www.pri.uk.com/ksf/index.html

Perhaps the Sabre 45 would suit. It also has the additional flexibility to add additional 12.5 litre side pockets to take the total to 70 litres capacity.

http://www.raymears.com/Bushcraft_Product/293-Karrimor-Sabre-45/

I would also recommend the Cross Bow system from Lowe Alpine as I still use a Lowe Alpine Contour Crossbow 50 and the Lowe Alpine Crossbow 65+ Expedition pack (which are now unavailable frown ) if you can find a retailer. These packs are very stable and comfortable.

The Berghaus Bioflex® back system gets good reviews being exceptionally comfortable and again gives a stable load carry although the C7 Bioflex® 65 + 10 Backpack might be a little to large for your requirements;

http://www.outdoorsmagic.com/news/article/mps/UAN/3098/v/1/sp/

http://www.nevisport.com/detail.asp?StoreID=0&Depth=1&ProductID=404002