Hmm. My GF prefers the Ikea one so far, partly because we can get it sooner (the others have a 4+ week wait, but I think we can manage that), and partly because we can try it out. Ikea's suggestion of adding a mattress to it makes me concerned that it's a bit thin, but it's probably worth looking at.
We have lots of charity shops, but none do furniture. I've looked at all the nearby secondhand furniture shops - not just once, but every couple of weeks; they rarely have sofabeds at all, and their sofas are, frankly, a bit crap. I've seen a fair few of them on Freecycle days before seeing them at the secondhand shop.
With fabric sofas and beds, though, I would be a bit wary of buying secondhand - don't really like to think of the years of piss and vomit that's been over them. If they were better value or looked OK I'd probably bypass this dislike, but it hasn't come up yet. I'd also be OK with a secondhand sofa from someone trustworthy (thanks for the offer, Pingu).
Many thanks for all the suggestions, you lot. I am a
very careful shopper, especially when it comes to spending this much money, so I'm not going to decide quickly no matter how much I'd like to get it over with.