From experience, I'm affraid there is probably not an awful lot you can do, other than pay it off asap.
The default will be registered on your credit file which will have an adverse effect on obtaining future credit, however once the debt is repaid it will show as being settled, but will stay on file for 6 years (i think).
You can put whats called a "notice of correction" on your credit file explaining what lead to the defulat in the first place...i.e you setting up DD and change of address, however credit company failed to do this etc etc.......this will be viewable by any future crditors you apply to and will provide them with an explanation as to why the default is on file.
If you look on the experian website it will provide further details......good luck