Disassemble left hand switch gear. (two cross-heads screws from underneath)

Seperate two halves. Note! pay particular attention to position of drip guard ring on right hand side. This is important for re-assembly.

Looking at the switch on upper shell, you should see three wires soldered to a plate. White, Black and Yellow/Black(May be Green on RSV).

The White cable is power to the Highbeam, the black cable is power to low beam, the Yellow/Black(May be Green on RSV) is power from the dimmer switch.

To keep the low beam on when switching to high beam, unsolder the black lead from its current position, and solder it to the Yellow/Black lead(May be Green on RSV).

Logic: yellow/black lead(May be Green on RSV) now provides power to low beam (via black lead) regardless of switch position (can still be controlled with dimmer). High beam rocker switch allows/blocks power to high beam via white lead as before.

Re-assembly. non-trivial. Two points to watch for!
1) Drip guard ring has a large block which goes to the front of the upper shell. It does not block the hole at the bottom of the lower shell as you might imagine it would. (That took me a while to work out!)
2) Pull clutch lever frequently to check for clearance and ease positioning.