I think i have seen a metal kit on e-bay from china or hong kong for about £25.
I have made a card model from a kit of one of the Kew Bridge engines. I have also made East pool engine house and Crofton Pumping Station.
I'll try to upload photos and a link later. Prices were £7-£11 and just stuck together with PVA glue. The beam even works on the Kew Bridge model.