Guys,
Thanks for the advice.
Decided I'm going to go down the "better safe than sorry" route and cut out a section of plasterboard and attach some additional framing between the studwork.
I'll take this opportunity to install the mains spur, and the media box cable and route these through some back boxes recessed to the correct depth - in the centre of the bracket.
I'll then re-install plasterboard and get it skimmed - as long as I keep the exact dimensions of the bracket postition I'll then be able to afix the bracket back plate to the new framework. I've done something similar for a big Italian vertical radiator thingy in the past and that weighed 100 kilos! so should be OK.
Thanks