If you have had no contact at all with the creditor of any agent of theirs for 6 years and the creditor did not get a CCJ within the 6 years then the debt is time barred. But it still exists - the barring simply means they can't take Court action against you. The debt could still be on your Credit Ref. Agency files.
Get your file from all three CRAs and check.
You say you think a place in Northampton "owns" the debt so you presumably have had some info about it. Northampton is the location of the central Court that processes applications for CCJs from a lot of creditors, so it may be that there is a CCJ.
N & P was taken over by another bank of building society - can't recall which one. They will almost certainly have sold the debt on by now. It is very unlikely they will have written it off, so your best bet is to find out who owns it and pay. If it is time barred (& there is no CCJ) you may be able to negotiate a part payment in full and final settlement.