I am really grateful for all the great feedback guys!
I tend to get a bit enthusiastic about new projects and rush in too quickly. I have to tell myself that there is absolutely no rush and, with this in mind, I have taken Andrew's advice and wll fit the dynamo and conventional bulbs before experimenting.
With that in mind, I went through my ice cream tub of old bulbs and actually found a 12v dipping headlight bulb with the same fitting as the original 6v one so that was a big bonus
Nothing on it to say the wattage just "BRITISH PRE-FOCUS" stamped on it but it didn't get upset when I tested it on a 12v battery!
The stop/tail light is a standard fitting so I have a 12v equivalent for there.
The speedo bulb is a bit more of a challenge. Small bayonet fitting and 6v 3w. I can find a 12v 5w that will fit but don't like the thought of losing even 2w that could otherwise be used by the headlight! I think I must investigate the back of my shelves - gotta have something!
Brian