Which multiplex? The two I can see that have indoor coverage of Orpington are South East London and NW Kent and South London. Neither of them have Caroline on.

https://www.ofcom.org.uk/tv-radio-and-on-demand/coverage-and-transmitters/small-scale-dab

There will be mobile coverage outside those areas, 59dbuv/m is required for that rather than 63 as well as some areas of indoor coverage.

I'm in the fringe area or just outside it for several multiplexes. Most DAB radio have manual tune. Set the channel of the multiplex, walk around and you'll then see the signal strength and when it's strong enough to receive it.