Adding to LAB's excellent and informative pic, here's what I needed to do to mine to get it to behave:
To dismantle the switch carefully tab the 4 bent-over parts of the housing back. Go easy here, only about 1/2 a mm will be required to remove the bakelite plate and spade connector stubs.
Be careful not to lose the 4 springs. It is quite likely that these are now loose in the housing and this is the primary cause of poor contacts. Use a large clean working space!
Wash all the bits down with isopropyl alcohol - removing all gunk, dust, corrosion and grease. Mke sure the nylon block is scrupulously clean.
Place a small drop of cyanoacrylate (supaglu) on each of the 4 buds that the springs sit on.
Drop the cleaned springs onto the nubs and allow to set.
Clean the copper switch rotor careflully with very fine wet and dry or steel wool. This too has to be immaculate.
Clean the brass contact rivets similarly.
Flush out the switch lock mechanism with CRC or similar. Blast away matey. Leave it switch entry down to drain out.
Get a SMALL amount of grease on a toothpick or piece of wire - generic will suffice if you don't have any dielectric grease - and lubricate the detent ball.
Work the switch mechanism 10 ~ 20 times with the key to make sure all is sweet.
Re-assemble complete mechanism - it only goes back together one way:
contact isolation disc on to bakelite contact disc
then copper switch rotor onto that
then offer this sandwich up to the nylon block - watch them there springs!
gently peen the retention tabs back over.