Ok, I replaced the original horn and wired them again using the original connectors, removed the battery for about 10 minutes and hooked up my GS911. There is a fault code on the EWS and cleared it. Unfortunately the bike still wont start and the EWS is still on the dash.
A mechanic replaced the EWS antenna, which he has done before so there should be no glicthes on the istall procees. Actually it was quite straightforward.
The EWS error can be cleared but once I run the diagnostics it comes up again.
Probably I'll need to recheck the wires again and make sure nothing is loose or cut. But logically shouldn't the horn wires have any effect on the EWS ?