I had a very good swim this morning.
I did laps for around 15 minutes, keeping a good steady pace the whole time. Then I did three 50m sprints (half/half crawl/breast-stroke), each of which was very near or below my previous personal best of 45 seconds.
Hard to tell whether they are the cause or if I'm just in good form today, but I made two changes to my swimming technique during the past couple of weeks:
One is "competition style" breast-stroke hand movements, as suggested to me by the friendly swim coach the other day.
The other is to spread my fingers apart while underwater instead of cupping them together, in both styles. My rationale for this is that the more skin comes into contact with the water, the more resistance I should have for pushing forwards. A bit like how a tree or bush will slow the wind down far more effectively than a brick wall...