At long last I've managed to do lengths of front crawl consecutively! The second had pretty scrappy technique but it was one. Then I repeated this twice more to make sure I could do it.
I'm surprised how long it's taken me to achieve this but really I've been finding 1 length quite tiring which is odd as I consider myself to have fairly good aerobic fitness. I suppose that swimming has far more restricted opportunities for breathing which puts demands upon tidal volume. Although I feel to be out of breath I suspect that there are regulatory issues from 'the central governor' perhaps initiated by fatigue of upper body muscles engaged in anaerobic metabolism which cannot be sustained for long. I wonder how many years it takes to get 'aerobic arms'? Perhaps only 10 weeks if you train 3 times per week according to this article.
Just to close, there are some interesting comments in here that relate fatigue to carbon dioxide build-up rather than oxygen deficit (increased blood carbon dioxide is a potent stimulus for breathing, oxygen lack is not) and ideas for bilateral breathing. That's something I need to introduce as the right is a preferred side already.