OpenReach call it circuit id. mine begins LLES then 7 digits. An engineer gave me it for checking my line on the DSL checker site, but it either only works with BT lines, or is different to "access line id" on the dsl checker. They also have something shown on the address checker called UPRN. If you enter just a postcode in the address checker, it gives a drop-down menu with every address on that postcode, including the UPRN. Entering this alone in the address checker returns results for your specific address.