Chris's reply about sums it up.
You will need a COC - certificate of conformity from KTM UK, (They may charge you for this... don't know), for the reg. process. It just details the specs of the bike and as long as the same model was sold in the UK, its good to go. Don't forget to get a bill of sale (Kaufvertrag) from the seller as I think you need this for the reg. as well
The only thing I would add, is DON'T declare the bike at the port of entry, even if its a new bike. It just gives you and the customs man extra work that is not nessesary... Do it all at your local registration office.
Its easy. More people should do it, especially for models that were not popular in the UK. You get so much more to choose from.
Me and a mate just bought BMW R100 GS PD's this way.
Nostradamus Ate My Hamster