Much stronger DAB signal...

[Gordon]

........... So far I have had pretty reliable results receiving DAB on it, except when passing Ashford Hospital on the A30. Going towards Bedfont it simply garbles for a few seconds. Going in the opposite direction (towards Staines) it not only garbles but repeats a few seconds. So much for the 'magic' of COFDM. eh?

Sounds like RFI to me. The repeat is interesting. Somewhere in the hospital [or nearby] there could be some 220MHz ish transmissions - perhaps a power line IT Ethernet system - that are interfering. Is it on all multiplexes? Does FM have the same problem? COFDM is pretty tolerant but not totally immune to broadband interference [and it does depend on the quality of the front end] that is sufficiently powerful.

Power line interference blocking DAB is well known and has been notified to OfCom several times over recent years but they choose to do nothing about it. People who sell these things also know about it but because they come in from China with a CE mark they are "OK" and so it is not the retailer's problem. Take a look at this:

http://forums.digitalspy.co.uk/showthread.php?t=1249767

The different responses going in different directions could be caused by the notches in the car aerial response. These are generally notorious for their uneven polar response.