Right! I think that I have figured it out myself. When I am modeling I switch between polygon and standard mode to select components - as you do - but because the model is getting more complex I got into the habit of hiding the subdiv surfaces to select and manipulate the polygon mesh more easily.

It appears that I was trying to select subdiv components in polygon mode while masked off and in doing so it was creating another proxy mesh with history...

I got around it and made the model lots smaller by going in to hypergraph and selecting the shape nodes 'polyToSubd1_polyToSubdShape1HistPoly1-3' and deleting them and there associated input nodes.

Worked fine and created a nice clean new polygon proxy mesh. There let that be a lesson to all you newbie Subdiv modelers out there!