Hi Ter, yes I agree, it may well be something else. I only suspect a little end as I can now hear a light rattle in time with the engine at idle that seems to be coming from the barrels/head area. On the other hand, the whirring and the heavy clunk I also heard when it happened would be more in keeping with gearbox or primary, or possibly the sprag clutch as it's an ES, but everything seemed to be working normally in those areas on the limp back to the garage. Starter works normally as well so still a mystery at the moment. I'll let you know when I have all the cases open and barrels off. Maybe I'll be able to postpone the full rebuild after all, on the other hand I may just bite the bullet anyway as I've been wanting a bit more peformance for some time.