In the end it was only necessary to hone the barrel to ensure the oversize piston fitted correctly. After collecting it from Damico Engines Ltd, at Roudham Industrial Estate near East Harling, Norfolk, I removed the old barrel from the D7 and fitted the newly-serviced replacement, That was yesterday.
This afternoon I completed the rebuild. Before filling the tank with petroil, I removed the tap - as it had been leaking - and dismantling it, dropped the cork plunger into boiling water for 20 minutes. After that time the cork had swelled and was a nice tight fit in the tap barrel.
After flooding the carburettor, I gave it a kick - yes, only one! - and the engine 'popped' into life. It was the first time I'd heard it sound like that, and it was so much quieter too, so I guess back in 1991 when I bought the Bantam it needed a rebore even then!
As it needed a new throttle cable (it broke on the way to the MoT station on 3 May, but I don't think I've told you that!) I've just ordered one from Rex Caunt Racing. More later, as it happens!
Have a great Bank Holiday weekend, all you Bantamites.