Man, took me a while to figure out what you are talking about here, but I think I got it (the 500 quid part clued me into the fact that the 'flat' you are referencing is your apartment - not the type of terrain you are planning on riding on)... You are looking for one of those indoor trainer things, your best response might be in the XC Racing and Training forum, I think more of them use those things than here in the all mountain forum - not saying that you won't get a great response here, but a higher chance of success there.
have you considered buying a good fluid training and using your regular bike with a slick tire? If you've got a road bike or a second bike, that's even better, as you can keep it set up for the trainer.
I bought a $110 Performance Travel Trac pro and use a 1985 vintage road bike and it's kept me spinning through the winter.
As a gym equipment mechanic/dealer there is one type of spin bike, though I am not sure on availability called the "X-bike" X-Bikes
They come up used sometimes for pennies on the dollar when gyms rotate inventory, such as the last set we bought out for $50 each . The neat thing with them is the resistance is grip shift controlled and the handlebar can move when standing for a little upper body/core use. Not the only option, but it came to mind when you mentioned your criterium.