I just ordered an 86 Chevette heater core (about 7" x 6" x 2" give or take 1/2") from autozone.com for US$25 shipped. All of the local autoparts shops wanted $30+ for similar sized heater cores, so this is a pretty good deal in my area.
Edit: sorry, just noticed that you're in England... Hmm, you could try doing a google search for places that sell autoparts in your area.
nat many places sell car bits online in the Uk, the best thing is to go down to a scrap yard and ask for a heatercore/heatermatrix. there is a guide on www.spodesabode.com on how to get one from a car yourself
I have a couple of heatercores out of some ford fiestas; Together they measure about 30 cm by 30 - 40cm. I stuck them together with chemical metal when I was trying out watercooling stuff ages ago. I was going to use a desktop fan in a box with these, away from the PC originally. Although I'd need to fix them up to seal them together and make them usable, you're welcome to have them if you pay the shipping / come and get them (I payed £10 at a scrappy and they're sitting doing nowt now). I'm in Norfolk if you're interested Splashback. I'll post some more details when I get home from work (night shifts at the mo) and include a picture of them at the moment. I haven't tried them in a "real world situation" kind of setup, but they look like they'd do pretty well. They do look a bit battered but like I say, I can tidy them up for you.
Sorry for the delay... Anyway, here's a pic of the rad. I don't know if the pic's clear enough, but it's a foot across the actual core almost exactly. I'll patch up the gap in the middle and make it a bit more ship shape if you want it.