Thank you for the advice and suggestions. However, as I'm in UK I've decided to first try what is available here so I've ordered a 2-piece front clamp unit (item B07T4KTZBD from Amazon UK) as it's both cheaper and quicker to get than something from Asia. If it becomes obvious that it won't work then I'll have to move to a more expensive solution. The photos with that product listing also illustrate your point about the cable routing from below - I had been thinking of the cable pulling from above which would be a straighter routing from the hole in the frame.

I don't want to be changing the crankset if I can avoid it. The current chainring on the outside of the spider is about 48mm from the centre which is not a very good chainline for the hub gear which is 42mm. Initially I am planning to try a 52T (ie existing chainring) / 38T chainring combination but, if successful, would like to substitute 54T to give slightly higher gearing when on the big chainring.