Thank you for the help, as an update I have received the parts mentioned before from
@Tam the Gun very kindly ordering and posting them to me. After disassembling the gun, drifting out the pins, removing trigger mech and replacing the plunger + spring on the magazine cut off I reassembled the shotgun to find it STILL had the same malfunction...
Not one to be deterred I disassembled it again and got my other Hatsan disassembled to compare against the offending shotgun. I noticed the bolt movement on the offending shotgun was choppy. I took apart the bolt, cleaned everything internally and externally, polished the mating surfaces of the bolt and the bolt carrier, applied a film of oil and now it works flawlessly. Poor old girl worked herself dry!
If a problem seems unsolvable, it's often the most simple thing going wrong eh? Lesson learned