With new 60/55W H4s in (the old ones were 60/55 too), and the clean reflectors, the lamps are much better - on the verge of being tolerable when dipped, and quite adequate when on full beam.

Until I took the headlights off, I didn't realise that there was a connector on the headlamp unit itself - it's much more usual on European cars for the wiring just to pass through a grommet into the lamp unit.

I'm still not happy about the 0.75 volts drop, and I'll make another measurement to the headlamp connector, to see if the bad connection is in the headlamp itself, or upstream in the engine bay or light switch.

Thanks all for the help so far - it's really making a difference.

Although the aim is currently OK, my headlamp positioner doesn't work. Where does the unit draw vacuum from? I have traced the vacuum pipe (pink!) back as far as I can, and it disappears under the heater unit to the passenger side of the car (RHD). What level of vacuum should I measure at the dashboard switch?