You'd done virtually all the diagnosis Ian, i only had to work out the logical conclusion. If the motor runs direct from a slave battery then the fault must be in the wiring or the switch. Next logical step is to jump the switch, if the motor works then it has to be the switch.
Had a look on fleabay, all of the UK switches for estates have disappeared leaving only saloon switches. While it would get you out of trouble for now, it's not a long term solution. I've been cosidering renewing mine as the "flick" function has never worked, the wipers work in the three normal (int., slow and fast) but no "flick" function. I've lived with it for 3+ years and was the reason i tried some contact cleaner in mine but i think i'll need a new one soon.
Have a word with Mark (griston64) as he's breaking a 98 940 at the moment, also Luke (360beast) has a 97 breaker but i can't think what year yours is at the moment so if those years are wrong then let me know and i'll keep looking.