Hi Vacek:
I would not even attempt to send it back to the factory, it would probably cost more than the gun is worth, or at least half. I would send it to Dave Slade, (
http://www.airgunwerks.com/). he worked for Beeman, back in the day, and is one of the best airgun tuners around. His nick name is the "trigger man," Because of his fine trigger work. There are other tuner's that you could send it to, but Dave has done work for me and to many other airgun shooters to mention. I would contact him first and tell him what you want, or think needs done.
He tuned an RX 1 for me that was the most accurate piston gun that I have ever owned or shot, and I have had and shot almost all of them, TX's and Beeman's, and RWS's and on and on.
I will have to say that I never owned a Whiscombe, but I have shot a few, and there is no way to compair a factor air rifle to one of those 2000 plus dollar custom wonders!
Jim