Andrew - I'm with Diyisfun on this one; IMO wall-mounted screens in lounges look pretentious, invasive and jolly uncomfortable to watch (they always seem to be mounted too high). The only, IMO, acceptable locations for this type wall mounted installation are; kitchens, offices, or modern open-plan (Holywood style) living spaces. And of course there's always the issue of accommodating the various Sky & DVD boxes, and the dangling cables ... commonly 'hidden' in surface mounted trunking (Yuk!).
Consider mounting your new telly on a floorstand ... not one of those, IMO, naff glass-shelf-4-chrome-leg-jobbies ... but something a bit more stylish (Google for TV floorstands or variations of this), there are 100's of great designs available.
If you really do want to hang the tackle on the wall, others here have indicated what to look-out for: match the weight of the screen to the appropriate bracket; fixed mounting or tilt & pan, or tilt, or fully adjustable ('broken-arm' type). Obtain an outlet plate (for behind the screen) with power, aerial-satellite, HDMI all in one.
You have a party wall so be VERY, VERY careful about drilling into this, or even chasing this out for burying cables. Do not assume that this party wall is 2 bricks thick + cavity; I've seen some built in the early 50's, when building materials were scarce shortly after the big bun-fight with Jerry, only being a single thick. Imagine drilling holes for Rawlbolts and chasing a channel in something so thin ... what will the neighbours say when you break through the other side? And while we're on about neighbours, you'll need their OK to do anything to the party wall.
Do send the letter 'up the chimney' to Santa but include the floorstand, then on Chrissy morning all you have to do is get the stuff out the boxes, assemble it, plug-in, lie-back with a sherry & mince pie to watch that old German woman's show at 3.
