Right, solution time.
Turns out I didn't need an extended bolt, washers, or a longer bolt. The bolts unscrewed (with difficulty, they were in there well tight) by the roughly 5mm required to do the job until sometime in the new year (-ish), when I'll lay my mitts on some wheel spacers for a more permanent solution.
FYI, spraying the both liberally with GT-85 three nights on the trot may well have saved me from shearing them - they're a very tight fit by the feel of it, and smacking them with a hammer a couple of times to shock them loose, then using a foot and a half long breaker bar, before laying on with the socket set was the only way to get them started even then. It's interesting to note that they did NOT want to come out all the way - the threads seemed to tighten up again the further out they were unscrewed, so it's just as well, I suspect, that it was only 5mm of additional length that was needed. Any more, and I think I may well have been in trouble! I've applied some threadlock to 'em in the mean time, and later on, once the threadlock has has a chance to set, I'll paint a thin white strip along one side face of the bolts, threads, and a bit of the steering knuckle, to let me periodically check in case they move.
Many thanks for all the comments above, they were very handy