So having let these calipers in California go past, I dropped by my mechanic today to see how my right side caliper, bought from Sussex (England) on eBay, was coming along (new seals etc). The two allen bolts which hold the caliper together were frozen in place from 27+ British winters of salt, something we simply do not experience in Australia. He's got one out, but the other one is putting up a serious fight. Maybe Californian calipers would have been a better buy!
Given how hard the right caliper is to find, I'm not prepared to sink money into the master cylinder and brake lines until I am sure the caliper is useable, so this is holding up getting back on the road.