Stephen,
I have been looking on the web at several options.
Gitzo G2272M is a top line precision engineered head with Q/R plate held in a dovetail slide. This means the Q/R plate is held with the maximum surface area. There is a price to pay for this level of engineering though, and W/E price is £164.99.
http://www.gitzo.com/
Manfrotto have a more agricultural look to their heads. That is not to say that their performance is inferior. You do however get what you pay for in ulimate stability as you progess up the range. Links to 3 different head images attatched.
http://www.manfrotto.com/webdav/site...zoom/141RC.jpg
http://www.manfrotto.com/webdav/site...s/zoom/029.jpg
http://www.manfrotto.com/webdav/site...s/zoom/229.jpg
Warehouse Express used as price comparison.
141RC Basic Head with Quick Release plate ( rectangular 200PL-14 ) £49.95
029 Standard Head with Quick Release plate ( hexagonal ) £69.95
229 3D PRO Head with Quick Release ( hexagonal ) £135.95
As pan and tilt heads have 3 axis, flexing is the key thing to look at. This will come from the style of Q/R plate and its method of clamping, and the general ridgidity of head construction. The pictures show significant differences in construction and materials used. The first significant change on the move up from the 141 is the mounting plate. This is carried through to the 229. Unless you particularly fancied it the 229 is a bit over the top for your requirements.
Straight race between the 141 and 029. Looking at the pictures the £20 extra for the 029 looks like money well spent.
Bearing in mind your preference for a head to take your existing Q/R plate, I have this morning conducted an experiment. This is the plate fitted to fluid heads 128 and 701. As I have the 701, I attached a spike to the plate and hung a 4kg load from it in any orientation that you might use the camera to check security of the plate mount. Nothing fell off.
The hexagonal plate would give a higher degree of security though, and suffer less from any twisting movement.
Don