I dont believe they can.....however it wont stop them giving it a go. The original lender/person you owed the money too will have sold the debt on to an agency.
there is no limit as long as the existance of the debt is estabished at least every 6 years. In reality they have probably sold on the debt as suggested above.
A debt company should normaly be banned from chasing the debt if you or they have made contact for 6 years and you have made no payments for 6 years. seek advise for CAB.