In uk.d-i-y, Ric wrote:

Thanks for the response! I contacted them and it turns out that they no
longer do the trunking range and that it is now done by Mita. I cantacted
them and managed to get a part number for what I want (CYB302W) but am now
having trouble locating them anywhere! It appears that most people order
the whole lot in for large projects and as I am only a small timer who needs
about 7 of them no one is interested! If anyone knows of anywhere that
might stock them please let me know!

From your descriptions it sounds rather as if RS (rswww.com) may be
your friend here - they sell a MITA-made 3-compartment dado trunking
range, with 2-gang backboxes as their partnum 563-696, and other stuff
in the range in their hierarchy as Electrical | Conduit/Trunking
| Dado & Skirting Trunking | Cableline Dado & Skirting Trunking

If not there, try www.farnell.co.uk. Both will do small orders with
card payments; you'll pay premium prices, but if it means you can reuse
the existing trunking you're on a winner anyway. May be worth buying
a few more bits-n-pieces as spares if you're likely to be working on
the room again some time in the foreseeable...

HTH - Stefek