Well I have taken all your comments on board, and have gone with Gavin John Hyde and AJShep on this. One trunking for mains to a socket behind the TV, another with 2x HDMI (neat, flat cables) a CAT5, 2x coax, and a nylon cord to pull up anything else I may need in the future. As to the TV mount, the old one was a single point wall mounted bracket with swivel and tilt etc but just not beefy enough for the weight of the TV which always sat slightly drunkenly down at one side. The new one isn't quite flat-to-wall as it has a tilt feature, but plenty stand-off from the wall to get a dual back-box in with brush plate one side and socket outlet the other. The new mount is rack- style so centring the TV is a doddle, and it is much more secure too.
Mind you, my arms are sore from putting that TV up and down so many times yesterday, checking clearances etc but I think it will be a reasonable job when done...not that the client will appreciate how much effort has gone into it, all she will see is a telly on the wall...
Right, must go and cough up some more plaster dust...