This evening I finally got round to sorting the plenum with a freshly purchased "round file" !
The pistol now shoots well but I'll now need to find out what pellet it likes. I gave JSB Jumbo Exacts a go as they happened to be handy but they seemed to to a wee bit slack in the barrel so something like H&N Field and Target Trophy 5.53 or 5.55 might work better, who knows ?