All sorted.

Massive thanks to Andy who quickly assessed my level of incompetence and did 98% of the job for me!
There are two earths coming off each light cluster, these need to be fed to the same pin in the multiplug to meet the single earth wire on the car loom. Numpty-bo11ocks here had fed each one to a separate pin in the new multiplug, which left us with a problem. Once the pins are in the plug, they ain't coming back out, as they have little one-way barbs on them to keep them in place. Not being able to remove the two separate earth pins I'd set up, we were (Andy was!) left with no other option but to discard the multiplugs and join the wires directly, which was done using crimped spades, wrapped with insulation tape once the functionality of the lights was checked and double checked. The bridging of the two clusters for the reverse and fog lights was achieved by running the two-core wire that Steve Morris recommended to me (purchased last-minute from Maplins in Bath, en route to Andy's 'cos I'm crap!) through a box section in the boot sill that handily happens to have a number of evenly spaced holes in it (allowing nice easy access to feed the wire through but keeping it safely out of the way) and splicing into the reverse and fog wires with scotch-locks. It all works perfectly.
As I said, I have to doff my cap to Andy for this, I genuinely couldn't have done it without him, as it turned out I'd aready managed to fubar the first part whilst supposedly following written and pictoral instructions! This wasn't a 'nice to have, just because' mod, but something absolutely essential to keep the car legal once we get to Germany. The man even fed me sandwiches, sausage rolls, biscuits and tea!
Rest assured, Andy, a bottle of something nice will be coming back from Germany with us for you.