In message , Woody
writes
One thing that everyone has missed is that DAB is vertically
polarised.

No we haven't. That's why I asked what the existing FM aerial was (but
you've snipped it all).

Thankfully all FM transmissions in the UK are now of
mixed polarisation (i.e. they have both horizontal and vertical
components) so turning your FM aerial vertical (if necessary)
will not be a problem (electronically - might be physically!)

As others have said a simple single half-wave vertical VHF dipole
is the best as (a) it is just about a full wave at DAB
frequencies so will work albeit with a mismatch and (b) it is
near enough omni-directional so will not limit you to your source
Tx station.

If the existing FM aerial is horizontal, to get DAB it would be better
if it could be turned vertical. It should still work well for FM. As you
say, its length will be a complete impedance mismatch for DAB, but it
might work well enough. If not, that's the time that the OP needs to
think about doing things a bit 'more properly'.
--
Ian