In case anyone's interested, a trip to Lidl earlier resulted in me spending a massive £4 on a spare bulb kit. 2x H4 halogen headlamp bulbs a double filament brake / tail lamp bulb, a dsingle filament brake light bulb, and some assorted sidelight bulbs and four fuses.
I know the headlamp bulbs will fit the disco, not sure about the others. It's a nice little kit about the size of a large matchbox, should fit in the cubby box ready for when it's needed anyway.
They had some larger boxes there too, although I didn't investigate what was in 'em. Maybe it's a spare bulb kit for those of you who need sealed beam units!
