Okay, in the end I changed the plan. Looking at the other tyre currently on the rear, it didn't have far to go and was also wearing unevenly thanks to being on the front with a crook alignment. I figured it might have 1000-2000km left in it, and seemed to me it wasn't worth the mucking around. So as an interim solution, I am having two new tyres put on now and two later. Not as good as four new ones, but at least if I do ever attend a cruise I won't have to show my car in public with the spare on Click here to enlarge

Now, to get that scratch on the bumper fixed...