Just thought i would mention I bought a black refill kit for a printer cartridge from Asda for £1, thought i would give it a go for a quid what have you got to loose, I have a HP Deskjet 3845 just a standard run of the mill printer.
Had a quick read of the instructions and basically you peel the sticker off the top of the cartridge which is stuck down well, underneath the sticker is a small hole which you put the syringe (supplied in the pack) into and basically squeeze the ink in, simple as that took about 2min and the kit has enough ink to fill the cartridge twice...
So a quid instead of £25.98 for 2 cartridges and it really is simple...