Posted this originally on u.r.c.maintenance, but only one reply so far.
May I try here? I see subsequently from Google that I can completely
hide one of these behind the dash (apart from the windscreen aerial).

https://www.dynamicsounds.co.uk/hond...-adaptors.html

I suppose it is the spread of DAB replacing VHF repeaters, but these
days VHF Radio 4 only seems to be reliable in towns, not in many of
the gaps in between. My car doesn't have a replaceable "head unit",
but has a USB power supply and a "line in" socket. Any advice on good
DAB add-ons? Or will I still find significant gaps in coverage?

Pure Highway 600. Also gives you a decent quality Bluetooth streaming
setup and handsfree for your phone.

We use one in the Toyota Alphard camper, to save swapping out the JDM
head unit.