Hi Julian ,
This is undoubtedly a freelance model and is probably an assemblage of parts from several other models . It is certainly not built to follow design of any full size engine .
You don't say really how big it is but that boiler looks very 'Maisie' like and those wheels are off a bigger main line type engine built to a smaller scale than yours .
The chimney on the other hand is from a model built to a larger scale .
The slidebar and crosshead system would have no credibility on a full size engine and on your model is probably just an improvisation .
Unless you bought this engine with full knowledge or at scrap price you may like to consider getting some money back .
Sorry that I can't be more positive .
Regards ,
Michael Williams