The nightmare deepens - need to replace one piston so took the sump off (eventually - who designed this damn engine....) to find the oil pan full of what's left of the oil pump chain tensioner (bits of broken steel spring & plastic foot)! The chain looks damaged also, like its been squashed / pinched!

Interestingly undoing the bolts that hold the balance shaft assembly to the bottom of the crankcase i noted uneven torques required to loosen - i swear someone has been in this engine beforehand.....

Anyone recommend somewhere i can get a decent reconditioned complete engine? Scoured a few internet sites but they all look a bit 'iffy' and i'm not sure i'll be getting any better than i've already got!