I want to put two single width (85mm) BS3676 switch plates side-by-side
on a wall.

I understand that a flush-mounting, 35mm deep metal back box is
available which is just a bit bigger than a double width back box and
has a metal strip down the centre of the opening. This strip has
mounting holes so that two 85mm plates can be mounted flush together.

Anybody know who is the manufacturer of this type of back box? Or can
anyone give me a part number and source, please?