This passed by my nose in the last few days. I believe it's a Stewart Riedpath 'Gnat' possibly marketed by Hamblings just post WW2. Heavy does not describe it. With a whitemetal body, possibly a lead chassis, and two bits of brass strip bolted to the bottom, it weighs more than most of Hattons Bachmann stock put together. Apparently it still runs well once warmed up.
I have a feeling that John Ahern used this chassis under at least one of the Madder Valley locos. I wonder how many of todays current crop of RTR will last seventy years?
It was passed to a friend of mine for disposal and has a boxed 0-6-0 companion should you be interested in this piece of model history.