I've not been around on AB all day, so I've only just read all of the posts above. Thank you all for your lovely comments.
I'm not too worried about the furnishings. My carpet is the ultra-cheapo one that the builders put in, just to make the place easier to sell, when they built this place about 40 years ago (topped up with some rugs, acquired from various charity shops, auctions and boot sales, for about a quid each). So the cats can't do very much damage there! The back of the sofa has long been in shreds (courtesy of various felines) and there's no wallpaper by the front door as that's also been clawed away.
I've found that it's what comes out of cats that can be the problem, rather than what they actually do. For example, years ago I picked up a broadcast-quality reel-to-reel tape recorder in garage sale for £30 (which was worth around £400 if it had been in perfect condition). I paid £100 to have it restored to 'as new' condition (which took 6 months because the specialist technician had to source rare spare parts for it). The day I finally got it home my cat sprayed it and it never worked again!
Similarly I had a brand new gas fire fitted. I used it once, the cat then sprayed it and it's never worked since!