Donwill wrote:
I'm not sure what it's called but I will try and describe it's
description & function .

A standard approx 85mm square white plastic surround, to screw into a
standard wall box.
Has a 3 position switch, centre off, momentary action, what I would
call "press to make, release to break".
Press up to make a contact, press down to make another contact. (change
over) Spring release to centre off position.
Or possibly two buttons mechanicaly connected.
This is required to manualy open and close a motorised vent.

It's difficult to search without a name for this type of switch.
Cheers
Don

Your best bet for a fuller flavour:

http://www.letsautomate.com/11471.cfm

You will need:

1 off K3631 WHI Cover plate
1 off K3701 Grid
1 off K4900 WHI Switch

Note: The switch is rated 250V AC 10A


HTH