Thanks for your response wetbear. I have been around Highgate ponds this summer, but only as I was passing through London (either on my way to or from Heathrow or Gatwick - usually have to stay in London the night before a flight, so have time to fill). Didn't get the chance to swim, although the water did look inviting (if a little murky!).
Here's a link to a couple of photos I took
https://www.dropbox.com/sh/aomdrmzpi...2UU7HvxMa?dl=0
Glad to hear you were wearing speedos - the only way to swim