The first question to you has to be have you received any other contact from the lender or their agents since 1994? If the answer to that is no and you are sure of that then write back to Hendersons pointing out the Halifax have adopted the Council of Mortgage Lenders voluntary code not to chase shortfall debts beyond six years after the sale of the property. Do you know when the property was sold?(posted 7986 days ago)If they are within the CML period then serve a SARN on the Halifax in order to obtain the information that will let you see how they have established their claim. Follow the advice on this site in the repossession section and be prepared to tough it out.
As to the Limitations Act I think we are still waiting for a landmark court ruling on the whole shortfall / MIG / simple / specialty debt issue. Keep coming to this site and read the forum and other postings.
The other loans etc. that you mention should by now be statute barred and since you have not been pursued for these then I would think you are probably ok. Might be a good idea to obtain a copy of your credit records from Experian and Equifax.