If that's not the early stages of a migraine I'll eat my hat.
I've suffered from migraines on and off since my early teens and I have learned that hard exercise can trigger them: both full-blown and (more often) just the aura stage that you describe. I have found that I can miminise their occurrence by taking care to be well hydrated and to make sure that I have had a snack before running. I had a rash of them last year when I started doing a hard interval session at lunchtimes but having a mid-morning snack made a big difference.
Many thanks for the advice. I had an extremely good run. I'm amazed that I could run off-road from Docklands almost all the way to my friends' house in Broxbourne. I saw very, very few people. It's not a route that I would recommend for a lone female!
I certainly needed the head torch for the last couple of miles though.
Many thanks for the help Sheeyut! You make it sound perfectly do-able. I intend to leave work at 5ish so I expect to be running until about 8. I expect it will be quite dark towards the end so I will take a head torch which hopefully will be enough to prevent me falling in the river.