Maybe answering my own question, but came across this, which may be handy for folks.
https://canalplan.org.uk/index.html
as I can't seem to force cycletravel to a canal route and tell me what the mileage is - canals of course notoriously take the long way round.
It appears to give you the canal distances between points - journey times not relevant of course as those are for a canal boat and I seem to remember that they are limited to 4mph.
Also shows facilities along the route - water taps etc.
Would be interested in folks' views on how accurate it is.