Hi,
Definately looks like the maker's name plate from the front of one of the Hibberd diesels - I have a photo in the gallery of Hibberd 0-4-0DM 'Rochester Castle' at Chatham Dockyard, showing the 'Planet' plate on the front:
http://www.railwayforum.net/gallery/...&searchid=8996
It's still a very interesting piece of loco history, but unfortunately it's not obvious exactly which loco it's come from! Not sure of a Doncaster connection either.
Tony