The full beam to dipped function was getting increasingly intermittent and would get stuck in one state or the other. I checked out the stalk switch and my trusty meter said it worked every time, so (thanks to posts here) I deduced it was the A+B relay. Fitted a 2nd hand relay and that seemed to fix it, but only for a week. Now it seems to have gone again I fear I am now looking for an intermittent dodgy connection or earth. The offending connection may be in the relay box - disturbing it with a new realay may have made it work briefly. It is not obvious to me how to release enough wire to get to the underside of the socket block in the relay box.
On a minor point, there is a plastic clip inside the relay box to hold the A+B relay in place. I have managed to let mine drop right down and can't figure out how to pull it back up - I ass ume it has ratchet barbs somewhere but can't figure out how to release them.
My Aero had the same problem afew years ago, wouldn't switch from mb to dipped and back (quite embarasing when you are driving at someone on MB and can't dip).
I checked the stalk and all was OK, but found that the connector block was very tight to the uderside of the dash and the pressure from above and was making a bad connection at the bottom of the block.
I modified the dash and block to get a bit more space and everything has been OK for years now (I have an early SRS car where the steering wheel can't be adjusted).