It sounds like something I had as a kid, only mine was Triang (c.1960 pre the Hornby takeover). I believe it was called a coal/ore hopper loader or similar. It worked in conjunction with hopper wagons which had 2 levers which engaged protrusions on the unloader - one lever released the chute on the wagon and the second closed it again as the train moved on. The set came with a choice of plastic 'coal' or 'ore' (same as coal, but red) in a rectangular sort of crate.
My mum used to hate it as we were always treading lumps of 'coal' into the carpet :-)
Not quite AIUI / remember it, as you imply Rovex took Hornby over in about
1966, they then used the Triang-Hornby name for the railway model side of the business but most of the old Hornby (Double O) models were never produced / directly / by Triang-Hornby  but were produced and marketed by another Rovex/Lines Bros absorbed company 'G.R.Wrenn' although they were listed within the same catalogue as the Triang-Hornby range .
 the ALI (?) LMR 25Kv IIRC was one exception, the tooling had been started by Hornby so Tri-ang took over when the merger took place and introduced it under the Triang-Hornby name.
 G.R.Wrenn survived after the demise of Rovex, again becoming a separate company, complete with the rights to the old Hornby 'Double O' range, in the late '80's the G.R.Wrenn company closed down (IIRC retirement) and the stock, tooling and rights were sold to Dapol.