I would certainly endorse dry firing the rifle with the forend removed. If the two stage trigger pull is restored it would suggest to me that it is likely the stocking up (woodwork) that is at fault. If it remains single stage then you will need to look carefully at the component parts that influence the trigger pull. The trigger & guard, as already mentioned, & also the height of the collar, the front edge of the cocking piece, & the sear. The rifle may not have been deliberately 'got at', it may just have had a replacement part installed without proper hand fitting. This problem occurs very readily when trigger guards, cocking pieces & forends have been replaced without due attention to the EMERS/Instructions to Armourers for correct fitting.