At/near the tills in Poundland today, I spotted what looked like pairs of refurbed Cannon/HP/Epson inkjet cartridges at £5. I didn't note any numbers, as none seemed familiar to me ( and I don't need any at moment, as I keep a set of two black & one set of colour on standby).
If I may suggest a mail order place, if acceptable under site rules, I'd suggest:-
http://www.printercartridges.net/uk2shop.htm.
I met the owner one day on a telecoms installation, whilst he was doing his day job in IT. He supplied ink to our resident's association for our newsletter, a lot cheaper and quicker than the large suppliers, and with a no quibble warranty.