Gals,

Thanks for all your suggestions, I really apprecaite them.
I'm using presta tubes, but am anally carefull about making sure the little valve release (the thing you unscrew at the top) is properly tightened whilst riding - and, when unscrewed, kept central whilst I pump up.
I've checked the rims to make sure nothing's ripping the tubes, and have stuck the tubes under water, but they don't reveal a leak - in fact, they won't pump up at all.
I think maybe I have gremlins in my tubes!
I'm going to investigate the pump - and will check the stem CorsairMac - my tubes are currently Specialized, which may explain something perhaps?
At the minute they seem to be ok, but worth checking out.