I think my null modem cable has either been lost or swiped from under my nose because I can't find it anywhere ...or I could have left it at a friends house when I linked our PCs up years ago. Don't suppose he ever thought about bringing this to my attention. Grrrr!!!
Anyway there are three computer shops within ten minutes walk of my house and I visited them all today to see if they sell null modem cables. One guy went into the back room and fetched a telephone cable, the guy in the second shop just looked at me blankly as if I'd just asked for a kg of kryptonite and the guy in the third shop knew what I was talking about but said he hadn't sold null modem cables for three years. Maplin sell them though so I'll have to wait till I go into town on Friday to pick one up.
Thanks for the advice by the way guys. My PC is quite new and has two 9 pin com ports so it looks like I'll need a 9 pin to 25 pin cable. I think an adapter will probably cost just as much as a new cable (£5.99) so I might as well just get a new cable.