A mechanical drive?
Mark
Not only was it a step on from wringers it was a step on from twin-tubs which were just about reaching their zenith in the UK in the mid 1960s. Its just a shame that Parnall were not in the same league as the big four (Hoover, Hotpoint, Servis and Rolls) at the time who were able to command big marketing budgets (particularly Hoover) both directly and indirectly (by product placement) that they were unable to get a market share that was able to support their existence. Remember too that this machine had a good deal of the UK version of the Thor Automagic in its DNA, so its mode of operation was somewhat similar. As far as I know this machine (and its fully automatic equivalent) were unique in that they had a suds saver facility built into the machine. As it has a solid tub, after the wash was completed the clothes were spun and the water was flung into the outer tub. If "Spin To Save" was selected the water stayed there, if "Spin To Empty" was chosen the water was pumped out into the sink. After this spin the washed clothes were removed and set aside, while the water was pumped back into the wash tub for the next load. Rinsing had to be carried out after. The water was also heated in the outer tub, it was not possible to soak the clothes in water while it heated up.
The switch has a mechanical element, but not in the sense you describe - the switching from agitator to spin is handled electrically, not mechanically. I have re-posted one of Ian's pictures below to help make it clearer.
The Selectromat switch is on the left had side of the picture, the actual control control knob is connected by a con rod to the switch about 1/4rd of the way down the left hand side
As I understand it the switch has two electrical layers (under the plastic cover on the left) - one layer selects between Off, Heater and Motor, the other layer selects which windings on the motor are activated, thus selecting between agitate and spin. The mechanical element is on the bottom and is a diverter valve choosing (I think) between connection to the empty spigot, refill the wash tub or just block it off entirely - I am not sure if the pump ran all the time when the motor was running, I think it was.
You can actually see the two hoses - the tub refill is on the right, the empty is on the left, running behind the Switch. It connects to the red hose to the left of the Selectromat switch running from the bottom to the top of the control panel where the drain hose to the sink was "plugged in". Very much a case of "why have a simple solution when a complicated one will do". You can see the the hose from the pump travel up the left hand side of the machine. By the way there is a corresponding pipe on the right for the fill hose , you can see where it enters the tub above the main terminal block - not exactly the best place for it to be!
When either heater or motor is chosen the thermostat or timer is in line before the connection to either heater or motor, the red cable running down on the right is to the heater, the multi coloured ones on the left to the motor. Within the control panel the red button is a heater overheat reset switch should the heater be switched on when the tub is empty and a neon to show the heater in use.
Of course, as it was semi-automatic the filling of the machine was controlled by the operator, not my any form of mechanical switch.
And we know so much about them by this stage that there is quite a fund of knowledge about them. Just don't take the Selectromat switch apart! 
