Step 1: Basics
The switch plate is secured by 2 screws the mechanism by a further two the mounting case holds just an earthing terminal.
The first thing I discoverd was that after removing the switch plate and
shaking out some plaster granules vigorously operating the switch freed it off, but having already purchased a replacement I changed it anyway.
Step 2: Dissembling
start by separating the cord connector and either undo the knot or cut the cord. The mechanism is spring loaded so it is vital you hold the parts together as you release the securing screws once removed gently release the pressure and remove the back plate at this point the screw terminals and fixed contacts will fall out. The cord and the rest of the switch components can then be withdrawn from the switch plate, the fixed cams in the swich plate can now be cleaned & inspected for wear. The rest of the components as they come off the cord. a plain brass washer that acts as a bearing surface preventing wear to the plastic switch plate, a brass cup washer that forms the other half of the bearing surface and holds one end of: the large spring, the brass conductor and moving contact. The carrier plate and moving cams, a small spring and finally the cord termination.
Step 3: Repairs
My problem appears to have been that plaster dust managed to get inside the mechanism during 30 odd years of use and fouled the mechanisum a simple clean was all it needed.
A broken or cut cord could be easily replaced .
This switch is a basic 2 pole on off switch and only rated at 6Amps externally it looks very much like a 16amp rated power switch (intended for switching immersion heaters showers etc). Should someone have inadvertently used a light switch for power it might be possible to clean and refurbished the contacts I'd only considered this as an emergency measure.
The fixed parts and most of the moving parts of a 2 pole on/off switch are identical to those of the 3 pole change over switch. whilst it is possible to reconfigure a 3 pole switch to a 2 unless you have good moving contacts from a broken 3 pole & at least 3 good fixed contacts it is not possible to make a 2 pole into a 3 pole.
Step 4: Reassemble
It really is just the reverse of dismantling. Set the fixed contacts into the back plate and set aside. If changing the cord start by tying either a figure 8 or reef knot feed the long end through the cord terminator make sure the knot is fully contained in the terminator and trim the tail off if required. then feed the rest of the components back on to the cord. Feed the cord through the switch plate & the cord connector and re tie the knot. tension the cord,position & hold in place the back plate test the mechanical operation. Then refit the screws holding the two plates together. The best way to do this is insert a screw in its hole. Apply just enough pressure with the screw driver to hold it in the head of the screw and turn it very gently anti clockwise (that is as if you are undoing it) until you feel the screw suddenly drop a fraction it has just engaged with the old thread in the swich plate now turn it clockwise and it up tight, repeat with the other srew. As these fixing screws are course self tappers you could just stick them in the hole and do them up, they might find the old thread if you did that but would most likely cut a new one and if removed several times you would quickly have a hole in the switch plate that the screws could not grip in. You can now test your rebuilt switch either with a continuity meter or a simple low voltage battery and bulb.(you could just reinstall it and test it in place but best not to)
Step 5: Salvage
If you can't fix your switch there are some salvageable bits. self tapping screws for hard plastic. a couple of small and quite strong compression springs the larger one at least could be just as spring steel wire softened re worked and then hardened and tempered aging. The cup and plain washer make a good low load thrust bearing.
The fixed and moving contacts have potential to be reused in other places as electrical contacts. The fixed ones being solid brass could also be used as low speed shaft bearings.