No promises, of course.
The fix for this problem is to rewire the two headlamp microswitches as follows:
Original wiring: One microswitch provides a ground for the Dip and Main relays (BR wire), the other provides a ground for the Flash relay (BP wire).
New wiring: One microswitch provides a ground for the Dip relay, the other provides ground for the Main and Flash relays.
The problem is caused by:
1/ The ability of the blue Main beam lamp in the tacho to provide a ground loop path for the Dip and Main relays.
2/ The holding voltage of modern relays compared with the original relays. Although I've never measured, the original relays must have a holding voltage of more than 6v, while modern relays will hold with less than 6v.
With standard wiring the ground wires from the Dim & Main relays are joined together and then go to a headlamp microswitch. When the headlamp raises the switch closes and both relays are grounded. When you switch the headlamps on 12v is supplied to either the Dip or Main relay, switching it on.
When you lower the headlamps the ground is removed, so the two relays are now simply looped together, electrically in series. If Dip beam is selected the 12v on the Dip relay loops around to the Main relay, through it, and finds a ground through the blue Main beam warning lamp in the tacho. This results in approximately 6v across each relay. This is enough to hold the Dip relay on. So the lights stay on.
If Main beam is selected the 12v on the Main relay loops around to the Dip relay but cannot find a ground path, so the relay switches off. Hence the problem only occurs with Dip beam.
I ran into this problem when I fitted modern, fused, Hella relays. Of course, both old and new relays need more than 6v to switch on, so switching-on behaviour is normal.
In my case I've wired the two headlamp microswitches in parallel in order to increase reliability - if one fails the headlamps will keep working (a link between BR and BP). So my fix was to fit a diode to the Main beam relay where the 12v line from the steering column switch connects (UB wire). Nothing fancy, I think it was rated something prudent, like 20v 3w.
Other ways to fix it would be to find some old relays, or to remove the Main beam warning lamp.
Nick