Just as an update I've played around with this most of yesterday and
concluded it's the DA that's faulty. Although not sure exactly what the
fault is.

I changed the DAB aerial back to the FM one - mounted vertically.

There are four tuners - two DAB, two FM, fed via a DA.

DAB now fine - FM noisy. Aerial horizontal DAB poor FM fine.

With the aerial vertical plugging the aerial direct to any tuner - fine.
Horizontal - FM fine but DAB marginal on only one of the DAB tuners -
other one fine.

So either the DA is faulty - although it appears to be increasing the
signal level - or one of the tuners is *transmitting* a nasty back up the
line?

I'm rather lost with RF. ;-)

--
*Two wrongs are only the beginning *

Dave Plowman London SW
To e-mail, change noise into sound.