Thanks for the suggestions, I think my 12Kg MDF weight is correct. Wickes quote 40Kg for a large sheet and I worked it out from that. Also Ikea wardrobe doors I recently fitted were 13Kg and they did not seem excessive for the 3 hinge arrangement provided. A Wickes veneer internal door is 30Kg.

However, as Rod says rigidity is the key and I am leaning back towards a hollow construction (with diagonal brace) using 3" wide timber skinned with 6mm MDF on the front and 3mm hardboard on the back. The TV weight on the front will now to some extent be counterbalanced by a DVD rack for 200 DVD's on the back!