Sam
Have you tried air arms - the only reason I ask is that most of the parts for the protarget are the same as the trigger family for the Ev2. If it is the part I am thinking of be prepared for a shock as well - I am pretty sure the last one I bought was about £35 quid!!!
Oh, and Air Arms won't be able to send spares out until next week as they are having a stock taking session, but I am sure they should be able to help.
All the best
Lee