I had exactly the same problem as @MWMan fitting the outer seal. The caliper manufacturer, ATE I think, have a special tool to press the outer seal into place for a large sum of money.
In the end I bought some rebuilt calipers and fitted them.
Chance would have it, I came across this on ebay the other day,
https://www.ebay.co.uk/itm/335300826861?_ul=GB
Hope it helps someone.