Natwest / cap quest

oldboy282

Hi I had a loan with natwest which i was paying on debt managment plan and then they suggested i do it direct which i have for about a year and 3 years previous through debt company. I started getting phone calls from cap quest last week refused there security questions as didnt know who they were and they wouldnt tell me who they were working for. was a little worried so checked my credit report to discover 6 defaults by natwest. Have phoned them to question y and was told need to talk to cap quest as they have passed debt to them she confirmed they have had all payments.
I moved house in october and notified them as they sent me a letter to new address but cause they hadnt updated maiden name said needed to contact so sent copy of marriage certificate. they havent updated details so cap quest are sending stuff to previous address.
Can natwest sell it if payments have still been made ?
Is my next step to right to cap quest and set up repayment plan with them ?
Thanks any help is much appreciated.