Are they chasing you ?
Do they know your current address ?
The 6 year council for mortgage lenders rule only applies to members of that.
And if they are a member and have not written to the person owing the shortfall,
then usually it means they can't enforce it.
The 6 years runs from the date the shortfall debt was created, which may be when the house was sold, although i am not 100% on that.
If you are not being chased, i would leave it alone.