Brief update, based on a call from the garage so this is the full extent of my knowledge - A valve spring failed, the valve dropped into the piston

, the engine didn't like it!
When it happened I described the sound as 'like dropping a teaspoon in a waste disposal unit' and it seems like that was more accurate than I expected. Waiting for Piaggio to authorise the new engine but it looks like I'm going to be on the Stelvio for a while yet.
Most annoyingly, having run-in my bike, I now have to run in the Stelvio (it only had 300 miles on, 750 now) and I will probably have to run in the replacement engine when it comes! Fair to say I do not own a motorcycle to ride around being all gentle-like....