Your car most likely has a Lucas 2 speed DR3 wiper motor and I believe the P/N on the motor is 75311. There are several PRS7 and PRS5 switches that will work with this motor but you need to scour ebay to find them. I have had some luck with ebay UK - in fact I purchased this switch yesterday:
Allard Humber Riley Sunbeam Multi Position Rotary Light Switch | eBay
I know it is not the original switch but it will work with the DR3 motor, accepts the same knob and fits through the same panel cutout. I have a Lucas .pdf file which is rather large that lists all of the Lucas switch P/N's that will work with a 2 speed DR3 wiper motor. If you PM me your address, I will print it and mail it out to you or if you can accept a 7MB file and want to PM me your e-mail address, I can send it to you that way also.
Good Luck