Hi Neil, I have been able to find out that Kingston Mouldings made a hull and had plans for this tug, they are still available through Models By Design and Loyalhanna Dockyard. Exactly where this model came from is still unknown as I would need to compare photos against my model to confirm it was a semi/kit.
There are plans in British Steam Tugs by P.N. Thomas and I have located a copy and await it's docking in my harbour. 
Regards
Roger