Checking across the battery shows that something is not up to scratch but doesn't show where.

This website is quite handy on dynamo electrics :-

https://matchlessclueless.com/electrical/lucas/testing-lucas-dynamo/

It's a simple test and will let you know if the unregulated dynamo output is OK. If it is then you've got to look further.

There is a fair chance that you've simply got a battery that has become defective.

Put two wires into the dynamo after removing the standard connections...

Next join the ends of those wires together...If your system is negative earth, connect the joined wires to the positive side of a voltage tester set to record DC voltage...

Put the negative side of the tester to earth on the dynamo body...

Start the engine and slowly increase the revs..You should see 10-13 volts with a good dynamo (unregulated)....

If your system is positive earth reverse the connections to the volt tester from those described at the outset...Ian

email (option): ian@wright52.plus.com

Thanks a lot Rik and Ian.
Will do and see what's up.
Cheers.

email (option): John.kater@chello.nl