Whoah, sorry if this has provoked disagreement

Basically the story is - I caught the rubber gaiter with the wire brush, putting a hole in it. (was
just about to stop to mask off the rubber, so I didn't put a hole in it - ain't that always the way?

) so consequently the caliper has been fully stripped down (that's piston out, seal out, the lot) and painted. Good, used bleed nipples are ready for going in, replacing the rounded old one. One of the slider bolts seems perfectly serviceable, the other has has water ingress , and corrosion approaching the sliding part. The brakes were very stuck on, hence the stripdown, and I couldn't discount the slider bolts being the cause. It's my belief that the corrosion on the lip where the smaller gaiter/boot sits over isn't a very good seal, so more water could/will easily get in again.
I'm being a right tight-fisted g!t where I can, the whole project is on a shoestring and minimum-wage budget, but never at the expense of safety, or cost in the long-run, if you all follow.
Total cost of the refurb so far
bleed nipples - free
Paint Enamel - £6
Brake discs - free (mate's unused ones)
Brake pads - free (mate's unused ones)
Wire brushes - already had them in
Vinegar - £0.44 (de-rusting the bolts etc)
total of £6.44 per caliper, I'm getting there!

So adding the slide bolts which should be good as new for many years (as I'll be regularly checking them annually, unlike previous owners

) adds just under a tenner to the cost. Re-using all bolts. I was getting replacement new, but opted for refurbing the old.