Well, I've just finished the capacitor fix and all seems well. I can adjust the brightness up and down smoothly on AC power forced into DC mode, though the full test will be in the car over the next couple of weeks.

Hardest part of this fix was sourcing the cap. I don't have a nice stock of such things lying around, and I felt fairly silly wandering into Maplins and asking for a single 7p component

Still, if it's fixed now, I'm happy. Perhaps it was the normal problems for me then! More FAQ readage before asking next time! Thanks as always for all the help/advice from one of the best enthusiast communities ever.