Not for me: I am very selective in what I record and rarely watch anything else.

A few points for you.......
If you jump up and down on the floor you should be able to tell if its a timber suspended floor, it will sound hollow, if its concrete you will know about it as it will be knee shockingly solid.
Carpet and underlay over tiles works well, let the carpet fitters know if it is over tiles as the gripper rods will have to be glued down. If its a suspended timber floor you can probably lower the joists, if no void to lower existing joists you can use shallower joists and reinforce each joist with flitch plates.
If you want to pick up the tiles I would highly recommend a floor Mutt www.screwfix.com/p/roughneck-big-mutt-pro-multi-use-building-decorating-tool-9-x-7/36715 great tool. A couple of years ago we had to pick up 82 tiled floors for Ramada Hotel Tunbridge Wells and replace with Karndean. The Mutt worked a treat. If there are some tile that are a swine use a hammer drill and a wide spade bit. Please protect your eyes and ears.
If the annex is a flat roof then your a wee bit stuck. If it's a pitched roof then I would take the ceiling down, place 100mm Celotex between the rafters and thermal bridge the ceiling with thermal plasterboard www.insulationexpress.co.uk/Product.asp?gclid=CNHCxMiP084CFQfjGwodKaQGuQ, obviously fit lighting as required. If it is a flat roof you may want to gun up the floor, if it is indeed concrete and lower it that way, watch out not to pierce the damp proof membrane.
You know my views of mains, Always take a radial from the house tails, and stay away from the house consumer unit.
If there is another floor above the annex then I would use a felt backed cement acoustic board backed with felt, can't remember the brand name at the mo, I should know as I have about £1000 of over stock, in our store, it will come to me.
There is loads more I could say but I would need you to inspect the floor / ceiling. There are often ways around things.
Please do shout for more help.

Thank you, these are most interesting and I've already had a conversation with Darren at SVS.  It seems they can take the weight of the Ushers and are inexpensive enough to take a punt on them.

I've thought about that, Ali. It's quite dramatic and on my mind as a future possibility.

Sounds like you are going to have a lot of fun with your new home Martin... are you planning on doing the work yourself to the listening space?

I would be leaving the tiles well alone Martin . If you're going to carpet over something it may as well be tiles . Good luck in your new home

Yes, I think a 'quick and dirty' setup with rugs will be on the cards. Better get the bedroom and kitchen done first, though

I saw a set of six on eBay for a very reasonable price and I have loads of Nectar points, so they have arrived and I shall keep them for the hard floor. This will get interesting

Thanks, Tony. It's going to be interesting putting it all in a new room. Your input is always valued.