Setting: The top left-hand corner of Wales.
Googled: "Ivor the Engine" plus "policeman" or "police"
Read the Wikipedia entry for Ivor. http://en.wikipedia.org/wiki/Ivor_the_Engine
Read the Ivor website. http://www.smallfilms.co.uk/ivor/
Asked various people who claim to have loved Ivor as kids.
Any further suggestions for places to search, or how to search, very welcome.
Instant answer: PC Gregory! Thanks!