According to this page:

http://www.crompton.com/wa3dsp/hamradio/antcalc.html

the half wavelength length is 63 centimeters

Are you saying that there is nothing to be gained in increasing the length of the damp peice of string beyond this ?

Assuming that extending the aerial is a non-starter then I guess a proper in-car DAB antenna is the solution. There are various different types available, here are two that don't require you to drill a whole in the car body:

http://www.caraudiodirect.co.uk/product_info.php?products_id=3688
http://www.caraudiodirect.co.uk/product_info.php?products_id=3687

They all share one thing, a length of coax that needs to be connected somewhere. I know one side of the coax goes to where the damp peice of string is wired, but where would I wire the earth to ? Do I need to find some where in the tuner to wire it, or eath it to the car body ?

Is this likely to work, or am I going to be foiled by impedance matching issues (a quick flash back to an antenna lecture during my failed EEE degree).