Try BMW's own insurance - they're still the cheapest for me. I was insured with Carole Nash when I had my R1100RT which was fitted with a Datatool Stealth alarm/immobiliser. When I bought my 12GS, I was told by the dealer that there were problems fitting non-OE alarms to the GS because of the canbus electrics! Yes, I bought that hook, line & sinker

It's probably complete bollocks. Anyway, Carole Nash said they wouldn't give me anything for the BM alarm as it wasn't bloody Thatcham approved
So I told them to stuff it and switched to BMW's own insurance which was a whole load of cash cheaper than the Carole Nash quote - even if I'd managed to have a Datatool of similar fitted to the GS.