If not too far away, try BSD in Peterborough Eye.
On their website under NEWS, they do write ups of "dreaded running problem bikes" they get into their workshop.
The seal broke on the fuel relay switch and let some salt and water in. Leading up to that the bike was stalling and the ignition felt like it was cutting out. This was completely random with no rhyme nor reason as to why.
Seems to get worse when I leave the bike for a week or so and then use it. Tends to do it more when I am between 70-90mph. If I roll the throttle off, when I put it back on it can induce this cutting out.
Put a new fuel relay on which appeared to make it better but then when the bike was left and I went to use it again, the problem was back.
Had a new fuel pump fitted - no cure so putting my old one back.
And now BMW just seem to be stuck on what to do next. The bike has HiD Conversion other than that, standard.
Brian Giles in Pembury, Kent (near Tunbridge Wells) has been servicing my GS for years and has always been great - free loan bike too. Recently fixed my FD. Do a search on here and you will find lots of good reports about Brians work. Ex BMW service manager now with a fully fitted workshop at home