Check dynamo brushes, regardless of what volt meter is saying. Clean up commutator where brushes contact. When you say the battery lead off, are you reading at battery when connected, then taking the lead off? Because if your then not getting a voltage at the battery lead, the dynamo 'ain't putting out a voltage, your'e reading the battery. Two problems come to mind, brushes as said and polarisation. Have you been fiddiling latley? :nono: Before repolairising, disconnect regulator.
Deatils on the net. Loose fan belt, or slipping pulley, and of course CLEAN all contacts make sure there tight. Dynamos are pretty bomb proof, regulators can be a pain. Clean all the relay contacts gently with a emery board, don't be tempted to fiddle with adjustment, if any obvious scorch marks on winding, I'd advise a proffesional rebuild. Have you an amp meter fitted? If so what's that up to?