BT Home Hub 5 (Type A) Software version 4.7.5.1.83.8.204.1.11
But, false alarm anyway, nothing interesting is happening. The firmware had updated and reset parental control settings on the router. The domain is on some blacklist apparently so it was redirecting to a page to finalise parental control preferences.
Sorry it wasn't any more interesting than that.
Edit: the reason it took me a while to figure this out was that the settings page it was redirecting to was nothing to do with parental controls!
I visited some other black listed sites <.< >.> and discovered the pattern, then dug around in the router settings to see what had changed. Disabling parental controls sorted it and I can now see the squirrel/chipmunk/unidentified rodent.