Guys - I've got an answer on this after much faffing around - direct from one of the main people at Parcelforce.
It's in an email to me with all the usual legal stuff about not reproducing it, so I will not give any of the details of the person or the email other than this key point:
"It’s accepted that air guns may or may not be the type with an air cylinder attached. The important point is that PFW does not carry anything pressurised as stated below. (Parcelforce restricted list) If the cylinder is attached to the gun and is empty, this is fine. Such an item (gun) needs to be packed extremely robustly and discretely.
Compressed gas ‘no’. Empty - if part of a gun will be fine. Definitely not permitted Internationally."
So there is still the issue of insurance which is stuck at £100, but we now have it in writing that an "EMPTY" PCP can go via PFWW on a 48hr express by following all their guidelines below, and knowing the pressure vessel is an "accepted" part of the gun as long as it is not pressurised.
https://personal.help.royalmail.com....lVMlElMjElMjE=
Select "guns for sporting use" to get info.
https://www.parcelforce.com/help-and...d-restrictions
Restricted items from Parcelforce
This is the first time to my knowledge we have had any clarification on this?
Result
James