Can I fix a better Ariel to my Dab radio, it came with that silly plastic wire,?

If you are VERY close to the broadcast aerial, it could be overloading your receiver. The last thing you'd need is an antenna to make it even stronger. Directional antennas do have the advantage of eliminating multipath interference but if the signal is too strong you'll just overload the front end. I'd suggest a directional antenna with an attenuator inline with the coax. Other than that, try receiving without any antenna at all. Since you're so close, this may reduce the signal enough to where you get better reception. As long as you don't need to receive any signals from stations further away, that could work for you. A mate of mine lives close enough to the mast that the guy wires holding it up is about 4 blocks away. I had to open his clock radio up and cut short the internal antenna to allow reception of any station other than the one right next to his house. It was just completely overloaded.