Thanks for the responses. I'm obviously not the only one to have the problem with the glueless patches. I tend to patch rather than replace the tube because I don't need to remove the wheel to do it. It's pretty quick and easy to pump up the punctured tyre and then listen closely as you rotate the wheel until you come to the place where the puncture is. Then I simply flip one tyre wall over the rim with my plastic levers and pull out the affected bit of tube, mend it and shove it all back in again.

My Merc Kevlar tyres have been rather good at resisting punctures. I ride a lot on bike paths that have tiny bits of sharp glass scattered around. Since nobody sweeps these paths much, the stuff lies around a long time if some drunken student on his way home chucks a stubbie beer bottle on the ground. I managed about 2800 miles with only three punctures and went another 800 before I got another. It was that one where the repair failed last week. Then my strida knock off got a puncture in its non-armoured rear tyre and that repair failed yesterday about a week after the repair was made. Strangely, that meant two failed patches in a few days.

Those patches are no good at all. I'll never use them again. The traditional sort work and last.