In the end, the combination of the time of day, the occasional spray and muck left over from gritting (even though it hadn’t been icy) and the way I drive meant it took 2½ hours.

The routes themselves were almost identical, the only difference being the route in or around Grantham.

And the other differences were:

1. the Garmin had the speed camera database to warn me of “accident blackspots”;
2. as the Nokia was on the cradle and connected to the car kit, every spoken direction muted the radio which is a tad annoying when the voice prompts get a little frantic; and
3. the Nokia’s display also shows the current speed (good) and the time left rather than the ETA (bad).

Looks like there’s still no ideal solution for me, but the Saga-driver Nokia is closest as it’s so nicely contained within the phone.