Ask the previous owner/tenant/landlord if you can, or maybe the estate agent, your conveyancer or the letting agency. Arguably, the pass number is as important as the house keys and should have been part of the handover pack. If it wasn't, your agent, conveyancer etc didn't do all the job he was paid for.
You'll have probably found by now from your own searches online that Melcom is a UK company and still active. Just in case it passed you by, their contact details are here:
https://melcomsystems.co.uk/contact-us/
The irony! That website address resolves to an insecure site, so may need you to intervene if your browser objects. You might mention that if you do contact Melcom.
In case you can't get on to the website:
Melcom Systems UK
Office No 2, The Giraffe House, Burrough Court, Burrough on the Hill, LE14 2QS, Melton Mowbray
01664 454 066
info[at]melcomsystems.co.uk
Tktnh
February 2025
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