Strange F20 routing

Something really odd has become apparent with my Navman F20 European version - how's your London geography?

I needed to travel from the West End (round Trafalgar Square) down to Margate. Both times the F20 has come up with a really strange route - it sends me a few miles WEST along the Embankment, then over Chelsea Bridge, via Clapham Junction and Lavender Hill to finally join the South Circular at Wandsworth, then all the way back round the South Circular to join the A2. Weird!

The correct route would be either along Commercial Road and through the Blackwall Tunnel or via the Old Kent Road. A route via Wandsworth is miles out of my way.

My best guess is that the map data that lists the speed for a road is corrupted in some way - maybe the satnav thinks I can travel at the speed of light down a particular road near Clapham!

The unit is up-to-date with the latest firmware and appears to function correctly in all other respects. My previous UK version of the F20 didn't generate such an erroneous route.

Anyone got an idea what is going on?