Stripped yet again and this time I had a good look at the valve cap and body. Some little brass hairs here and there so I gave it a clean and polished the spigot. Reassembled with some light oil but it is exactly the same!
I'm wondering if the valve cap may be sticking in the transfer port sleeve (or whatever it is called!).
I assume the chamfered opening of the port sleeve goes towards the back of the gun and that it acts as a guide when the valve is being closed by the return spring. Can't tell from the exploded view.
Anyway, new seal kit on order from USA so I will probably wait.
An update on this. Seal kit received yesterday and fitted today - all working properly.
Interesting to note that they actually supplied a proper quad seal and not an o ring for the valve cap.
Somewhat disappointing that the gun was bought (from here) as working and as having had new seals recently.
Thanks to everyone who's offered advice, and yes, it was the quad seal after all. It's just the seller's assurances that the seals were new that threw me off the scent.
Cheers Simon