I have checked/replaced that one. The actual supply relay for dip beam is in the fuse box. The blue relay under the dash is connected to the earth path of the off-side headlamp as you say.

I think that all the switching/relay system is now correct, it's just the light output that's insufficient which will be due to resistance somewhere. I try'd a direct earth from the near-side unit with no improvement so it appears to be on the supply side.

There is not a great deal to the supply side when you break it down, 12volt supply from battery, relay, fuse for each light, bulbs, earth.

I shall dig the multi-meter out and attack it over the weekend.

Cheers.