Good news - granddad has confirmed he had the sludge trap cleared out "a few years ago" (could well be 20 years, but at least it's been cleaned at some point).
I have another oil question, which I'm sure will get a different answer from everyone - which oil should I use in the gearbox, and also for the chain? Again, the Haynes manual suggests 20w50 for gearbox, as in the engine. Im guessing there won't be a simple answer and everyone will have their own preference, but I'd be interested to know whether I should use the same as in the engine.
Thanks,
Sam.