Thanks for the info. I'll speak to a dealer about that.
Well today I stripped it down. First check was coolant level:

Phew all OK.
Next check air filter:

Not too bad (was supposedly changed at a service at 11,056 miles on 05 Dec14)
Next remove air filter to check breather hose...


Holy crap!! Well that explains that! So thoroughly cleaned the sponge, including decreasing and drying. Cleaned out the air box - fortunately the sponge contained it. Ran the engine at idle for 15 mins. Re-cleaned air box again. Ran engine at idle for 20 mind, then went through the rev range for a few minutes. Re-created and reinstalled everything including a new(ish) K&N filter (thanks Zippy) and ran engine for another 5 mins.

I think it's starting to clear a bit though am concerned by the amount of crap uncovered today in the air box. I'm going to check it out over the next few weekends in advance of a trip to Scotland at the end of the month.
Am still concerned by the amount of crap in it- stuck my finger in the oil filler opening and inside seems caked. I might buy some Motul engine clean and change the oil/filter again (only changed 200 miles ago and the dropped oil didn't look too bad).
I'm also concerned that my last service was at 14,858 miles on 06 June 14, which means given the air filter was changed 6 months before that, and a big chunk of the riding would have been summer/autumn, this has supposedly occurred over this winter which was relatively mild... (just hit 19,000 miles). I'll have to keep a closer eye on this in future.