I see you're a fellow Northamptonian, try Northants Welding Supplies in Wellingborough 01933 224614, they are pretty helpful, or AT Tool Centre in kingsthorpe, 01604 711711, they both have trade counters, Failing that B&Q keep spares for the DIY type welders, probably best to call them contact tips, if you get the wrong person on the phone they'll think your asking for advice!
Paul