In securitisation, backed by a pool of mortgages, notes are issued to investors - The ISIN numbers are identifiers for the notes and hence can't help you identify the mortgages included in the pool. Since the notes are usually listed on a stock exchange (LSE or ISE most commonly) you can find some details about the securitisation from prospectus - however they won't tell you which individual mortgages are included. The list of mortgages is usually in a document called "Mortgage Sale Agreement" which is not public. Can't think of a way to get access to that.

MAS number 5, is a company owned by Platform Homeloans which is a 100% owned subsidiary of Britannia building society. Most probably WMS would be servicing your mortgage.