That is a terrible answer - if you leave a property because it is repossessed and move to another property which you own, either on your own or in conjunction with somebody else - any equity you have in that subsequent property can be siezed in bankruptcy or through a charging order as a result of a judgment.(posted 8127 days ago)The former mortgagees are clearly onto the case - they are after a shortfall and they will continue to try and find you until 6 years is up.
You have two choices basically - ignore the letter and hope you can continue to avoid county court proceedings for six years and then you are safe, or face it and respond to the letter with your circumstances, but note that the six years starts again if you acknowledge the debt.