All done and appears to be back to normal. Removing hands free/kill switch block is a piece of cake. No special tools required, just a long thin allen key.
Tried taking the old block apart to see what's in there. Got the two halves split but there a tiny screw to release the inner workings which I can't get to with the jeweller's screwdrivers that I've got. But it looks like there is absolutely zero in the way of sealing or water proofing. Seems crazy to me that you'd put delicate pcbs in upward facing motorcycle switchgear without out any kind of protection to keep them dry.
I'm glad mine isn't a tourer that gets ridden in all weathers otherwise I'd be riding with my heart in my mouth waiting for the bike to die every time I got caught in the rain.