So.........I replaced them this late afternoon, early evening. No real problem with the wiring or the drilling of the holes (the replacements dont match the exisitng holes), but to fit the devils I had to remove the metal plate that hides all the wires, and to remove the plate I had to remove the back seats!!! Since I'm replacing these anyway, that wasn't a total pain.
What a faff! Still, the new lights are very bright, and quicker to light than the old bulbs, they look great.
As an aside, I'm building up a pile of things that I need to get shut of.......if anyone has a use for two 3-man bench seats in decent nick, or a front centre seat in reasonable condition then pm me. I've also got a shire blue grill and headlamp finishers. You'll have to collect but a very very nominal fee is all thats required (only for the seats, the grill and trims will cost you nowt).