For everyone who's been so kindly supporting me over this build, some good news!
Attachment 34156
All of the strings are perfectly centred over the slugs -- the bass E looks a bit off in the photo, but that's just parallax.
For the amusement of those more able than I am, and the possible education of fellow newbies, here's what I did to get it all straight this time.
Following Simon's advice earlier on, I had kept the centre hole and only dowelled the side ones. So, I screwed the bridge in place with that hole and lined it up with the line I had drawn along the nut side of the bridge when I did this previously and messed up:
Attachment 34157
You notice that the line is a couple of mils away from the bridge? I drew the line along the edge of the bridge last time around, so it should have been flush. I was puzzled about that for a moment, and then realised that the reason I messed up last time wasn't because of my crappy drilling. Well, not *only* because of my crappy drilling.
Last time, when I put the bridge in place in order to work out where the side holes should go, I used the centre screw, just like this time. But I'd left it a bit loose so that it was easier to pivot the bridge from side to side to get it just right. What I'd forgotten was that the hole for the screw is bevelled so that the screw head can be flush when it's fully done up. Which means that when the screw isn't reasonably tight, the bridge as a whole can move about 2mm forward or backward. When I put the bass and top E strings in place to check that it was straight, even the minimal tension on them was enough to move the bridge forward that 2mm -- which meant that both of the marks I'd made to drill were 2mm forward of where they should have been. No wonder it didn't work!
(That said, the hole I drilled on the bass side last time around was more like 3mm forward and 1mm off laterally, so there was drilling incompetence involved as well as incompetence in marking where to drill. But even if I'd been drilling in the marked places, it could never have worked.)
So, I tightened the screw as much as I could while leaving a little play so that I could pivot it to set it straight, put the bass and top Es on again, and started the drilling again. This time, it was all manual tools. Firstly, I marked the centres as well as I could with the 0.5mm drill bit, then put that bit in my new £7 pin vice...
Attachment 34158
The vice showed its discount nature when I tried to use it with the 1mm bit, though, and kept slipping. No matter, on to my other new toy:
Attachment 34159
It was pretty pricey for a hand drill, at just short of £28, but worth every penny. Really solid, no play in it -- it just works perfectly. Very very happy with it. Amazon UK link for anyone who's interested (though in the light of the problems I've been having so far, perhaps I'm not the best person to be giving tool recommendations).
So anyway, I used that to work my way up in half mil increments doing each hole alternately until one of the holes was 3mm diameter, then put the screw into that one, and finished the other and put the screw in that one too. And it all worked perfectly.
As I was a bit concerned that, with all of the dowelling and drilling and redowelling and whatever, the bridge might not be very strongly attached, I decided to string the guitar up fully with the baling wire that came with the kit, and get it in tune so that it was at full tension. If the whole thing was going to fly off I'd rather it happened before I spent time finishing everything. I did so (as per the photo at the top) and left it for three days. The only issue was that the top string parted ways with its ball end (ouch) when it was somewhere around a C, so the nice Ernie Ball one was brought into service.
Not a creak for three days, and when I dismantled everything today, all looked well:
Attachment 34161