Jbwheel, there are no Gamba importers in this country, more's the pity.
Paul Thornton at North Humberside Shooting Ground (01964 544357) used to be, and AFAIK, still has contact with Gamba in Italy. Might be worth giving him a call.
To get a spare trigger group isn't a simple "buy it, slot it in and shoot" exercise. Each trigger group needs to be set up especially for the gun it's going into. If not, you could face double discharging, second barrel not firing, and so on.
People's assumption because of this is that the guns are not reliable, but this isn't the case. If the trigger group is balance correctly, it will live forever! The only problem I've ever had with mine is when I tried to use the wrong trigger group!! (Now there's a kick in the shoulder...!)
I have a spare trigger group, and one day when I find the time, I will be taking it to the factory to have it set up.
If you fancy a jolly to Hungary in June for the European Fitasc, Enrico Gamba will be there (with his usual two gorgeous assistants!) I will be getting my gun serviced there.
Let me know how you get on with Paul.