Everyone's missed the obvious solutions ...
a) Get a smaller telly and sit closer to it. You'll see the image the same size as you were planning to.
b) Buy 5m long M8 stud and a similarly sized drill bit and drill horizontally until you break through to daylight. Fixing to an external wall should do the trick.
c) Purchase recliners and ceiling mount the b*****r. Joists are easier to find than stud and far stronger. Wear a helmet whilst watching the box just in case though.
d) Buy bean bags and put the mother of all TVs on the floor. You may need to strengthen the floor to cope with the point loading though.
e) Currys claim they'll install so test them out. If they cock it up and "upstairs" becomes the new ground floor then you'll know who's insurance to claim against.
Thanks for the cracking suggestions, nice to see someone put a lot of thought into their answer.
I did suggest a solution similar to "B", however SWMBO though the neighbours had enough Coat Hooks in their hall.
I am going to go for a combination of dado rail behind the partition and 12mm plywood behind the telly.
If that doesn't work I will let you know, unless of course I was coming down the stairs at the time of their collapse and I am laid out in the hall under our new mezzanine bathroom suite.
Richard, I should add that I did not mean you had not put any thought into your previous answers.