Air leak FTP900

[hitNmiss]

Hi Guys first time on here, advice needed plz, it is in reference to my former FTP 900, I bought the the gun around 3 yrs ago, I couldn't fault the Rifle, but because of work etc I was unable to commit the time to using it at my FT club, so thought instead of it wasting away on the shelf, let someone else get the benefit of a top class rifle so sold it last year. BUT now it has started to lose some air via the the fill port (only slightly), is there an easy fix or would it be best to take it back to Air Arms for a service as well ?