How to turn chased in back box to surface box

So I have a 3-gang light switch chased into the wall. Only one switch being used. 
I need to bring this out into a 1-gang surface switch plate so I can get trunking into it. 
the room is grade 1 listed so can’t go chasing as it’s also a hospital. Basically going to make it 2-way. 
I can’t just fit a single box on the 3 gang chased box as it’ll leave a massive hole. 
many ideas on this one?

Put a timber patress over box fixed to wall. Hole in middle to bring cables through. Happy days. 

A 1G switch fits the same size box as a 3G switch so just fill the surface box with cement or plaster and fix the surface box.

Or are you proposing an architrave switch?  Same applies fill in the old box hole and fit the new box.

Some three gang switches use a 2gang box. 

Double sized plate with a Click paddle switch in middle......3 module plate, but paddle switch may be only 2 module

the switch front plate fixing holes are the same distance apart on all 1 gang plates, so the flush back box and the surface box have the front plate hole's in the same place, i would punch out the surface box taped holes and use long 3.5 mm screws from the front plate to the flush back box 

poni said:

the switch front plate fixing holes are the same distance apart on all 1 gang plates, so the flush back box and the surface box have the front plate hole's in the same place, i would punch out the surface box taped holes and use long 3.5 mm screws from the front plate to the flush back box 

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The flushed back box is a 3-gang box so a 1-gang surface box wouldn’t be able to do this

bhamoggy said:

The flushed back box is a 3-gang box so a 1-gang surface box wouldn’t be able to do this

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I'm confused...what do you mean by a "3 gang box"

boxes are usually singles 75 x 75mm or doubles that are about 135 x 75 ish

bhamoggy said:

The flushed back box is a 3-gang box so a 1-gang surface box wouldn’t be able to do this

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I did not konw they did a three gang back box for switches, I have seen them for sockets, is it,  a three gang socket or is it a switch? 

Normally three gang switches fit onto a one gang back box ,unless you use a grid type switch then it's a two gang or twin back box 

Whatever the actual sizes, put a flat metal blank on the larger flush box, cut a hole for the cables, and screw the surface box on with two self tappers.  Or am I missing something?

Geoff1946 said:

Whatever the actual sizes, put a flat metal blank on the larger flush box, cut a hole for the cables, and screw the surface box on with two self tappers.  Or am I missing something?

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I don't think you are missing anything ,it is that simple 

Why not save a lot of hassle and work and use Quinetic switches & a receiver... get whatever grid frames and plates to suit the backbox

NozSpark said:

Why not save a lot of hassle and work and use Quinetic switches & a receiver... get whatever grid frames and plates to suit the backbox

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Tried that but the light is a 60w 1200 x 600 panel and drivers won’t dim it that size

As asked before

how big is this box in mm ?

Only 3G box I have ever seen was for one of those 3gang sockets with a built in fuse

kerching said:

As asked before

how big is this box in mm ?

Only 3G box I have ever seen was for one of those 3gang sockets with a built in fuse

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It’s a 3-gang grid plate. 1 switch and 2 blanked out

bhamoggy said:

It’s a 3-gang grid plate. 1 switch and 2 blanked out

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Why do you need to fit a single backbox?

If you only need to get it to take trunking then,

Why not fit a twin surface by knocking out the taped holes and useing long 3.5mm to hold the grid in place 

Or a twin skeleton backbox this would  do the same job

poni said:

Why do you need to fit a single backbox?

If you only need to get it to take trunking then,

Why not fit a twin surface by knocking out the taped holes and useing long 3.5mm to hold the grid in place 

Or a twin skeleton backbox this would  do the same job

Click to expand...

Ok, so if I did that, and put a twin surface box in, what’s the best answer to give me one switch in the twin box? Would a grid frame fit?

Yes, why not it fits a twin box now why would it not fit a new twin backbox then 

bhamoggy said:

It’s a 3-gang grid plate. 1 switch and 2 blanked out

Click to expand...

Right, so it's NOT a 3 gang box, it's a double box

so,why not get a dimmer, get a double blank plate

drill hole in blank plate

fit dimmer

or am I missing something ?