I spotted a large oil puddle under the engine [2L air cooled] and found that I must have lost some sort of plug from the back of the engine block next to the oil refilling pipe. Please see attached photo. Can you tell from the photo which part I have lost and whether this indicates an underlying problem [blockage, ...] with the oil distribution system.

It will be the main oil gallery plug,there are a couple of them and they work loose over time and then pop out,I tap the hole and fit a plug.

Thanks, and there are no other parts, springs, ... sitting behind the plug that could have popped out?

it is the gallery plug as described No!.The oil pressure relief valves are accessed from below.From the pic I can't tell. ( a couple of beers etc)

Interesting, the one time I should have been watching the oil pressure and the oil temperature gauges to pick up a potentially "catastrophic" lubrication system failure, I didn't. Pure luck that I lost the plug just before parking the car [must have, otherwise there would not have beenthe large oil puddle] Yet to repair and check whether the engine has copped it. Hope not. Thanks for your advice. How about sparkling?