rich imho you will end up with the tv on the ground with plasterboard fixings
it sounds to me like you live in a timber framed house and it may have a double plasterboard slab to achieve a certain fire rating.if you can locate 2 uptsanding studs,use a studfinder[available in b&q etc] cut out the plaster board between the studs which are usually at 400mm centres but can be at 600mm centres.
you can either cut the plasterboard halfway on the stud,or fix another batten to the inside of the stud.
fix your plywood to the stud i would use 3/4 inch ply and then screw your bracket to the plywood and hang your tv.

cheers james