Hello Gareth,
if you read my articles in the P6 NEWs (feb 2002 page 7 - tells of my fitting the later model 75 watt inner lamps in place of the original 37½ watt as I was assured that the standard wiring was perfectly suitable to upgrade to twice the wattage on the main beam lamps by Steve Hopkins the club technical advisor on electrical problems) then in October 2003 pages 12 & 13 describes my fitting of the Lucas Halogen headlamps (the ones with HALOGEN written across the front that a lot of Triumph Stag owners fit) and the wiring mods required to have both lamps on with main beam setting using a changeover relay - The Rover inners have two filaments for dip / main whereas the Lucas lamps have single filaments which work fine with the Stag but need a modification on the Rovers.
The P6 NEWS of December 2002 page 19 shows a small correction to my October article.
Headlight conversions using standard halogen bulbs of 55 watts do not require any modifications to the standard wiring, assuming of course that the wiring is in good order - The lamp glasses will get a lot hotter than lamps using tungsten bulbs but they do not draw any more power.
I hope this helps - I know that your Dad, or Alan Pinder, will have these magazines.
I bought used Lucas HALOGEN lamps from a club member for a very reasonable fee - You can still buy them new from Rover / triumph stockists such as Rimmer Bros. but they are very expensive, or you may be lucky to pick some up on e-bay, as I have also done!
Regards, John