Thanks for the pointer. Pulled the oil pump out & just as promised there was the seal half way down the shaft. Now clearly that's fine when it's the open end of a condom, but in this case a little silicone gasket & I've tucked it back in place. Once the gasket's dry I understand I have to bleed the pump before I reassemble the lot?

The search feature & I aren't quite friends yet, so are there any tips for this? I saw mention of a bleed screw in the other post. Is this on the side of the pump that faces the centre of the bike. Is it simply a matter of opening the screw & rotating the shaft to pump some oil through?