Hi Neil,
Not sure if you have found somewhere suitable for refurbing your calipers yet but I just got a quote for refurbishment of 2 x Beta Spyder calipers from "Brake Caliper Solutions" in their standard paint finish (silver, black, gold) for £129+VAT. I believe they also upgrade to a higher quality finish for £169 + VAT.
Their blurb states:
"All stripped down & blasted back to bare cast, finished to which finish you require, then new pistons and seals. Bleed nipples are used."
Not sure whether "Bleed nipples are used" means that they reuse your old ones, or just that they actually fit bleed nipples?
https://www.brakecalipersolutions.co.uk/