If the gun is not held on a Shotgun Certificate here in Britain it does pose a slight problem. It needs to be back on a certificate before you can proceed further. Get your relative to take it to a good Firearms Dealer who can accept it in. Provide a plausible reason for not being accounted for and he can enter it on his books and notify the local Police that it now exists. Guns turn up all the time like this having been found in attics when people die and that sort of thing. Despite all this gun registration in Britain there will be more that are not on the data base than are. There is not usually a problem by adopting this method. DO NOT take it to the Police first as this WILL complicate things as they do not have the means to accept it other than as a surrendered item which will probably result in its ultimate destruction. I have helped with quite a few guns that have come my way having been found when people pass away and I have managed to get them back quite legally on certificate through my local Gunsmith. Lagopus.....
