Thanks everyone for your help a few days ago. Just had time to fit the replacement unit, it's replacement pulley ( ! ) and fan, then repolarise from -ve to +ve earth. Touch wood it all seems to work ok and has run 10 miles without going smokey inside the cab or throwing rubber around under the bonnet. Happy man so far! Will stay with this dynamo now that I've splashed out £29 on it!
Eddy