Thanks for the tips guys - I'll start the hunt there.  It'd be really nice to not have to pick the exact moment to turn the wipers off!! 

75Van
Just on this (I'm looking at upgrading the switches in my van too) - do all Mk1 wiper motors have a self park function?  At the moment I have to flick the switch off when the wipers get to the right position! 

Has the washer foot pump in your car been removed? If it has that may be why the wipers are not self parking.
 
 

It has - the only foot switch is for the high beam.  The washer pump is activated by a momentary switch, which certainly looks aftermarket.  Is there perhaps a wire or wires that should be connected or isolated to make the wiper motor think that the foot switch is still there?

Not familiar with the foot switch (mine only had a water pump bulb to wet the screen then I think you had to manually start the wipers) but if it is automatically starting the wipers AND the water pump then it would be a momentary switch that grounds the low speed circuit thus activating the wipers.  The existing self-park system would then take care of returning the wipers to park once the foot switch is released.
 
Either pick yourself up a replacement wiper motor or take your apart and clean the internal contacts.  You may even have a broken wire inside.
 
Ignore the photo of my wiper motor.  I was going to tell you what each wire did but I cannot find my diagrams.  Also, nobody else ever had the same colour wires as me!!