Thanks for pointing that one out on OSM at the top of Sullart Street. Interesting, I don't believe that one exists anymore - possibly because it was too close to the delivery Office on South Street. I will check next time I go down there which won't be long.
As you suggest, maybe it was moved to Fern Bank (before my time in Cockermouth) hence the same number. I'm sure the Fernbank number is correct though.
The Royal Mail site shows the Fern Bank one (with that number) but non on Sullart Street:
https://www.royalmail.com/services-near-you#/results?&postcode=CA13%200DF&latitude=54.659575&longitude=-3.365788&searchRadius=40&count=7&officeType=PB&filters=&selectedName=FERN%20BANK%20D
The link you gave is a good one for checking them!
Thanks again.