1938 16h dynamo

... the field windings are not being energised. Does it light a bulb if you join D&F at the dynamo and connect a headlight bulb between that & earth?

ps what sort(s) of regulator?

What is a standard Lucas Dynamo? E3L, E3H Or? Not that it makes a lot of difference, but knowing what you have is a good starting point. This 18V at the D&F?? Very little meaning. The D is the dynamo (armature) output, while the F is the Field input. The F input is a variable supplied by the Regulator and depends how much is coming out, ie voltage and charge current, that is the whole point of the Regulator it controls the Field current which in turn controls the D output.
To properly test a dynamo do NOT bother with a Volt meter.
First polarise ie take wire a from your battery LIVE to the F terminal for a few seconds on removal there should be a small spark. Use a head lamp bulb, the one with a single filament left! 12V is preferred but 6V if you like. Connect D to F and this point to one end of your bulb the other end to earth-start the engine. At a fast tickover you should have a bright light. IF the lamp is 45W then you have 45W.
Then you may connect your regulator into circuit with the bike and its battery. Do NOT worry what the ammeter is doing. (easily miss wired) Checking the battery for a rising voltage is the only really worthwhile check. NOT with a modern digital meter as they jump about with the ignition, an analogue meter (waggy pointer).
Electronic regulators? I sell one with Fitting instructions (as above), which WORKS. Other products might not be as good.   aoservices.co.uk

Just to add a little something to Al's comprehensive reply.
Direction of rotation is crucial to a working dynamo. Not all dynamos are the same.....
Don't ask me how I know.....
Duncan.

If you look at Rogers original question he had an output (albeit NOT measuring the battery as that needs to be on the output of the regulator) So direction was correct.
Duncan-direction of dynamo rotation? Looking inside the dynamo we have a Field circuit-two wires. And two brushes (these take the power from the Armature). The relationship between these two 2 wire items is crucial for correct working. When you have a dynamo repaired, they will ask you 'what bike or what direction' (if it is wrong-Duncan then there is no output unless you reverse your engine!!) Some bright spark now points out there is an arrow on his dynamo! Put there by the manufacturer 70 yrs ago. Now irrelevant. 
IF this relationship is wrong-no output, so you change around ONE pair of the connections/wires. Repolarise, check for output as above and carry on.
By the way Lucas failed in the 1980s the name is used by a non technical company who get things made in the far East Some things work sometimes, some don't.
If one regulator works then the bikes wiring is correct.