Pandora wrote:

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?

Most of the manufacturers have this item in their ranges. For example
MK list no. 887 ZIC dual flush box (also 888 ZIC dividing screen for
dual flush box, for where you need segregation between the two sides).

--
Andy