Thanks Guys,
you've put my mind at ease.
I've managed to pick up a bargain recently with a couple of sd cards. And then the horrid thought struck "What if the cards only have a limited shelf life of a year or so because of batteries or some other reason I hadn't thought of".
It would have been awful to not get any use out of them, but the price was too good to miss out on.
Thanks again.

best wishes
Rikki