From Muppet Wiki

The Captain was the British equivalent of Doc in the UK co-production of Fraggle Rock. The Captain was a Scots-accented retired sailor, now active as a lighthouse keeper, who lived in on an island somewhere off the coast, and who received Uncle Traveling Matt's postcards through a passing mailboat. The Captain was a bit crustier than Doc, and tended to be sterner towards Sprocket, but the two still had a very close relationship.

When actor Fulton MacKay died in 1987, John Gordon Sinclair replaced him as the Captain's nephew, P.K..

The Captain, along with Sprocket, appeared in the 1985 Royal Variety Performance. He gave a brief talk to the audience, explaining that the stories about strange creatures living under the lighthouse were utter nonsense. This was followed by a segment featuring the Fraggles.