On a serious note: I wouldn't contact them until they make it absolutely clear who they are and what they want. If they do this then you should serve an SARN on them straight away and follow the steps in 'Do's and Don'ts' under the repossession part of the website.(posted 8217 days ago)I can imagine how worrying it must be with the new mortgage you could well claim that you didn't admit to the repossession because it was so long ago that you were sure it was written off...which is probably what most of us thought anyway!
Be strong.