Thanks for those responses: Nope, the cert has not expired, it is brand new and the dates on it are correct - I even got another one to check that it wasn't just the cert and that I hadn't made a typo in the passphrase.
This is a new install for me - it seems, though I may be wrong, that the main problem is that I should have something in the Trusted CA tab of the cert manager - I have nothing. Does that spark any lights? Cheers Steve