On close inspection I found that one of the new flex coupling nuts was fouling the back of the top water outlet where the fan clamps in. The nuts supplied by Mike were double thickness, and there was no clearance, so the nut was fouling the water outlet casting and preventing the coupling, and therefore the entire engine from revolving. Phew - nothing serious!
I ground a tad off the back of the water outlet, and all is now fine. Of course, the M Types won't have this problem, as they have no fan, but something to watch if you are fitting new flex coupling nuts to a Minor.
