After much toing and froing have ended up with an ebay Chinese special
https://www.ebay.co.uk/itm/313949787916. This has a 14mm bore but 20mm pivot to abutment distance. For various reasons have only just managed to try it. The feel is much better than the original but not as soft as the last one I tried.
A brisk 20 miles showed me that the front brake is much improved both in feel and effectiveness but still needs a substantial handful to get a decent effect. All in all the mod seems to have been worthwhile and I enjoyed the ride very much.
Just one point about the ebay item - although it was listed as being located in the UK it actually took around 10 days to arrive and clearly came from China, so not one to use if you're in a hurry.