Have asked on Ixion (bike list) reasonably anonymised and the general feeling is that it has dropped a valve. The follow up to that is that it's probably best to replace the engine rather than strip it apart and rebuild (the more BMW orientated posters have suggested it will _probably_ be OK to just do head and piston because the lower engine is pretty bullet proof but even they have to agree that the cost of a second hand engine from eBay probably means it'd be cheaper to replace the engine.) The thing to look out for is if it has holed the piston. If it has then bits of metal could be anywhere and it's much more likely that it'd be more economical to replace the whole thing than fix.

Suggestions for mechanics have been -

"I use Bob Thomas in Finchley, the mechanic (Joe) is a nice guy. But they don't do engine work, so he'd only be useful for a swap."

and

"
If they are able to get the bike to Twickenham, I'd recommend:
http://www.ljmotorcyclerepairs.co.uk/
It's run by Les who used to run the workshop at Blays of Twickenham."

Which were from people I trust although I can't directly vouch for either company. I did ask for personal recommendations not google searches though.

I would offer to help move the bike around, but thanks to the LEZ my truck can't be driven within the M25 anymore...

Best of luck with it.