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This ones beaten me and I can't figure out why. Problem is that I can only get a dip beam to light up, the main doesn't come on. Its a DC wassell conversion with a dry battery, as I have done with all of my other scoots. So now I have the advantage that I can measure voltage without plumes of two stroke.

So if you look at the picture below, with the bulb removed, I have put the black prong into the side earth tag and the +ve prong onto the left hand tab and I get 12 volts. This is the same reading if I put it onto the tab at the base of the bulb holder, or on the end of the bullet. On the RH tab the reading is zero, but that is what i would expect.

But when i flick the light switch to main beam and put the +ve to the RH side, I again get 12v +. So it appears that 12 volts is getting to the bulb holder and is going to earth and that the switch is doing its job. But wtf is the bulb not lighting up on main beam. Note, I have tried several bulbs.

Aaagh help

Why not extend the "live" contact with a free lead and hold it against either of the bulb terminals and earth the bulb by holding the side against a known earth or again extending the earth with a free lead. Must prove/eliminate the causes of the problem.

As above or bend the contact in to be sure they are hitting the blob of solder on the bulb. I often add to the factory blob as they seem to cut back the amount of solder they use. Also the bulb can twist too far missing the contact.
Measure the continuity of the bulb. Swap the red and blue feed in over.

Dry rivet joint between the bullet terminal and the sprung arm

I'm with Wack. It's really surprising how often I've had a 'dry' joint cause a problem. All looks good but a continuity check says different.

had a duff joint in mine with same symptom

MK Monty wrote:.....Swap the red and blue feed in over.

Great bit of fault finding advice, see if you can 'move' the fault

Can't believe it; solved.

Tried all of the above and did think of dry joints, so got the soldering iron out.
This morning, connected up to a spare lambretta holder and same issue. Swapped a few bulbs, including halogen ones and same problem.

Then found a halogen bulb with foam on, tried that and all works. Now it might be that all the previous bulbs were dead, but why would I save them, probably because I'm stupid, as my wife says.

Anyway, just got to find the rest of the inline fuse holder that I carefully put down yesterday

But thanks for the input

You have gone to the trouble go getting a volt meter.. Set it to ohms about 7 o'clock on your dial. Put the tips of the probes together it should read close to 0 that's what a good lamp should read.. Separate the probes your meter will flash dashes or show nothing. That says no connection or a blown filament. Test your lamps. 1 probe on the outer case and the other on the lead solder blob. 1 will be high beam the other blob will be dipped. Simples. Try some lamps and maybe some fuses. Get used to how the meter works and it can save you hours of frustration.