Hi Ray,
I went down a similar route and bought a cheap aerial. This is the actual one:
http://www.ebay.co.uk/itm/290951505122
I mounted it under the plastic boot trim. (Sorry don't have photos of it installed)
It works ok. Not perfect as would the correct amp for digital radio. When in a poor area the DAB goes to 1 reception quality and then automatically switches to FM. This is unnoticeable when listening. However it's great 90% of the time. And for the little money it's ok until I do the correct amp job.
All I can suggest is to pull the aerial from the DAB unit and see if FW, LW etc are back to normal. If not I guess your new DAB aerial maybe inducing noise to the other existing aerials, as I'm guessing you installed it on the rear window?