Thanks for the replies.

I think it could be a case of me not tightening it enough. I found this particularly hard work by hand, I couldn't get the capsule to pierce using hand force alone and had to very carefully lever through the hole in the end cap. Hence my suspicion of the capsules I have. I tried two different ones and it was the same with both. Once pierced I then got the slow leak.

I have stripped the pistol including the valve assembly. There was a small amount of chewing gum consistency brown crud in the neck of the valve housing where the valve should seat. I have cleaned this out plus the rest of the components and relubed where appropriate. I'm going to pick up a few different brand capsules tomorrow on my lunch break and have another test when I get home....

Thanks again for the comments.

Andy