cheap vs expensive mattresses - false economy?

[Y0rkshirelass]

I'm looking for a single mattress for a guest bed, and didn't want to spend a lot of money as it will only be used occasionally. On the other hand I don't want guests sleeping uncomfortably!
Has anyone bought a cheap mattress recently that was good quality? For example there's one in Argos for about £40 but I'm worried it'll be thin and crappy.
Any thoughts appreciated.

For an occasional guest bed I would go for a more middle of the range one, for the reasons you describe.

Message withdrawn

don't spend the earth on one that will only rarely be used- you can always add some extra padding with a mattress topper or a spare duvet under the bottom sheet- makes a real diff!

Ok, I'll have a look at the medium ones. I might try a shop where I can actually fell the mattress, rather than ordering on-line.
Nearlythere - that's a good idea, thanks.