Sounds like this is fairly common. I've found even since I posted the original question, that provided I just push the card until it clicks, it starts up every time. With my old Axim x50v and Atom, I had to really push the SD cards as far as possible to engage the lock whereas the Dopod clicks just after the edge of the card is past flush. Gently does it seems the rule.
Mike, I appreciate your concern but the clunkiness I think is more related to the card design, having the bumps along one side, than the slot. Also, these things are all rated as hot swappable, so we should all not have to turn off just to change a card. I don't change cards that often, but when I travel, I swap cards a bit as I take a couple cards with music on to listen to while flying.
For all with the issue, I suggest you do as I said in the first paragraph as that seems to be the clue, just press the card until you feel it engage the lock and see what happens.
Thanks all for your feedback. I'll carry on without a visit to service as the phone is terrific and the keyboard (my reason to replace the Atom) is superb.