Thanks for the replies folks. I did a repair prior to my last trip which involved some good clear duct tape, and the inside has adhered really well, but the outer (which i'd done to stop any ingress to open the tear) took a battering from the rain, and the grainy muck is getting into the edges. Its a big tear on both legs which goes at right angles, so it needs a big wide patch to do it in one go.
My mate had also suggested the spinnaker tape, which looks just the job, but I'd discounted it on account of wanting something wider. But an Ebay search this morning following on from Tartanferret's suggestion, has thrown up a roll 75mm wide which might just do the job. That - and the clear tape on the inside should make a decent job of it I reckon.
I've also spotted a roll of black tape in my Farmers supplies shop which is about 100mm wide!. I'd been told of this before. Its used to tape up the holes in haylage bales when they get eaten into or split, so its heavy duty, but the ripstop nylon sounds the ticket.
Next stop will be sending them to Scottish Mountain Gear if all else fails. I'd replace them but there seems to be a dearth of good waterproof trousers about at the moment and mine - a pair of Mountain Equipment ones are top drawer. Just a shame I ripped them post holing through the snow in a storm. Don't you just love crampons for that! The irony being that I was wearing them over a pair of trousers with kevlar reinforced ankles for just this eventuality.
Many thanks for the offer btw Tartanferret
