Even though Haydn now has the parts I've sent him he's still having problems so I said I'd post on here what does what. :-
Attachment 321370
First off, the part you've circled saying is still in the gun has the exhaust valve sliding inside it, this part needs to be knocked out (forwards) to clean it, and I fit them onto the valve before inserting it all as a whole.
The exhaust valve is the brass cap on the back of the main valve and has a small O ring around it, there is also a small quad seal inside this cap that seals the air inside the main valve when pressurised --- if this has perished you'll not build pressure up.
Changing the quad seal is a bit of a fiddle but it's not worth rebuilding the gun without changing it --- hopefully Ped can supply you one.
The other part I sent you to the left of the part circled is basically just a sliding sear which the sear sticking upwards from the trigger unit catches to hold the exhaust valve closed in its forward position --- on pulling the trigger the exhaust valve is moved backwards by the air pressure in the valve allowing the air to go through the transfer port into the barrel behind the pellet.
Anymore questions just shout out.
All the best Mick