As 2.5 km was fine on Tuesday I tried 3.7 km today. Not good, The knee became an irritation.
The physio mentioned this little muscle on Monday and here's a little summary. It seems like the history, presentation and causes fit. I've had tight hamstrings for as long as I can remember and this is given as a predisposing factor. Notably I gave them a good stretch before setting off on Tuesday but didn't today; historically I've not been a supporter of pre-exercise stretching. This little video is quite useful.
As the muscle is involved in medial rotation of the knee (lateral rotation is the key to breaststroke) perhaps swimming is not ideal as there must be side-to-side loading. I've been doing 50:50 breaststroke and crawl, perhaps I need to adjust that.