I found some references online to something called "Smart Setup", which you can apparently turn of under Advanced Settings -> Home Network -> Smart Setup, but no idea what it actually does and why it intercepts random requests...
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.
the fact that it encodes "/" on a part of the url in a parameter, but not on another is a very good indication that whatever this 'feature' is doing is badly thought out and the implementation was done by the intern.
"Deployed on one out of four residential gateways globally, Cisco Videoscape OpenRG is the industry's most widely used residential gateway software." -- Cisco
At least we know a little about the software issuing this redirect!
The router itself is a BT Internet (UK) branded one. Not sure of the exact model but I'll try to find out...