Tennents Trains, Hayley Green, Halesowen, West Midlands, England, B63 1DY;-
Model train shop, but sells all sorts of all gauges/propulsions
New and used Mamod steam engines, fuel and spares, sometimes Wilesco (some spares available).
Secondhand stock varies, but has included Bassett Lowke and gauge 1 steam loco's and Hornby 3.5" gauge Rockets.
Metal, materials and cast fittings also available.
Oh, and Mamod of course http://www.mamod.co.uk/