And the winner is............Johnr.
The exhaust header had a leak. Weirdly, it wasn't sending stuff out, it was sucking air in. Must be some mad effect related to the head having the emissions watering down paths (reed valve removed and blocked). My tame tuning genius sprayed it with some sort of leak detection fluid and sure enough, slam the throttle shut and the blue stuff went IN between the header and the head. Two new studs and a gasket and it's service as normal.
Andy
