I've just had a response from HP's support desk. It's pages long. Shame I can't print it out........

Thanks Fred, but the link doesn't work.

I'm considering getting an HP cartridge tomorrow. Is that wrong, do you think?

http://welcome.hp.com/country/uk/en/support.html goto the hp ,uk and let it find the correct driver just put in the printer nr and type it will do the rest allways a problem with hp they have no drivers for vista yet

err, just a thought vicki, your new cartridge, is it a recycled one? has it got ink in it? does it feel heavier than your old cartridge, cos if not, it's not been refilled !!

Thanks Fred *hug*. Did a check on that site, and it says I have all the latest drivers. I did install a patch that was available for a scanning error - can't do any harm eh? - but I still have my printing problem. I may reinstall the printer software completely and see if that helps - that's what HP recommended on their 5 page solution........

Yes Denis, there's ink in it - the tissue comes away nice and black

Going through the advice from HP, I've just printed a test page, which failed on the black print. HP are saying if the test page fails, I should try another cartridge. Baz is going to go to PC World and get me an HP one......

I spent £48 on cartridges from IJT - if the HP one solves the problem, IJT can have all of theirs back and I shall be seeking a full refund!

I've just ordered a refill kit. I just can't bring myself to pay over £30 for a black ink cartridge.......
How do they work please? Do you inject the ink back into the old cartridge (that's what I'm guessing, anyway)

To be honest Vicki, it's always best to stick with the manufacturers cartridges. If anything, for the warranty alone. You may find that your printer warranty will be invalid if you don't use genuine HP cartridges. I know that's what we tell our customers at work. Hope you get it sorted.

there's a sticky label on top of cartridge, peel it back, you'll see 1 or 2 small holes, thats where you inject the ink with the syringe provided in refill pack. REMEMBER inject the ink VERY slowly or it'll overflow. there's a sponge inside the cartridge, you'll feel resistance when you push the needle in, insert all the way. i would only inject 5 ml to begin with, see how you get on. replace the sticky label or the ink will leak from print head after you refit it. remember to wipe the printhead as i told you last night.
PS: refill over the sink, just incase you have a spillage. ( get this right and you'll never buy another new cartridge again ) !!

Well, the HP one works (it was £15.99, so not as bad as I thought) so IJT are in for a rocket. Mind you, I've had them for over a year - maybe that's the problem.
I shall try refilling the HP ones when the time comes - thanks for all the tips Denis, and thanks to everyone for their invaluable advice xox