Many thanks to everyone. Just went out to look, but I've now done the
deed. I looked in Curry's, but that was hopeless, then walked along to
Comet, where there was a Garmin Nuvi 360T for £118, which was at least
as cheap as anywhere I'd been looking at on the net. Bought and took it
home to find that the battery seemed to have lost any desire to live,
there was a missing mains psu, and the unit was totally blind to
satellites, even when put on the Disco roof away from the trees.
Took it back and stood at the three tills alongside two others returning
a faulty vacuum cleaner and something Philips that had totally the wrong
thing inside when they opened the box. My young lady assistant poked at
the till and called another across, saying I'd paid £100 too little. A
small meeting was called, and they rejected my suggestion that they
refund what I should have paid. Then it was discovered that several of
the price labels were wrong in the display cabinet. Finally, I walked
out with a Nuvi 300T at the same price, which, not quite as big a
bargain, but again seems within striking distance of good value.

I've spent the afternoon and evening testing, updating and driving.
First impressions were that the display and finger access were good, but
the voice was absolutely awful. A female dalek would be mellifluous by
comparison. Some words like "In" sounded like a croaking click. An
update seems to have cured this, and I'm now pretty happy. Not tried the
traffic updates yet, though.
It's very sensitive, and worked when stuffed down in the Disco ashtray,
which has to be excellent. It's also of a size that will be easy to put
next to the old bolts and spanners in my coat pocket when leaving the
vehicle.

I'm really grateful for all the suggestions here, and have followed them
all up and thought long and hard. I very nearly went for the £44.99
suggestion, but wasn't quite brave enough.