Need to buy a cheap mattress - any recs?

[mabelbabel]

We've nearly finished entirely re-decorating our house. One last thing is to complete the spare bedroom. We already have a second-hand double-bed frame to go in there, but we need to get hold of a new mattress. The room will probably only ever be used for a few nights at a time for visiting friends/relatives, or may occassionally be let out on a short-term basis.
My usual default is to buy from Ikea, but we have no way to transport a mattress, and the 39 delivery charge is a bit steep if we're buying only one thing (I don't mind so much if it's a van-full!).
Can anyone recommend an on-line shop, or specific mattress, with cheap or free delivery? Medium-firm or Firm. Ideally we don't want to spend more than about 120-150. Is it do-able???

Do you not have a car? They come rolled up in ikea and very transportable if you do have even a small car.

Yes, we do have a car! I hadn't even thought about it, but yes you're right the smaller/cheaper ones often are rolled up (our giant one was not!).
I wonder if they would send the packed dimensions if I asked, or whether that info is on the website. Will check.

Even if they're not, you can rip off the packaging in the car park and roll it yourself if you bring rope/buy ratchet straps in Ikea. Much easier!

Yes, website says that it is rolled up "for easy transportation". Thank you!

I've bought from both matteressesonline.com and mattressman.com (i think they're called) and they were quick, cheap and efficient. Think they do next day delivery as standard too. Is it an Ikea bed? I think Ikea bed/mattress sizes are slightly different to standard ones.