Hi Norman
I had this problem with a French car which I had for resale. I could not source the rubber grommets anywhere, after a couple of sleepless nights and a few trips to motorcycle parts shops searching, I was about to give up, then finally after rumbling around in my garage I discovered the bulbous end of a tubeless valves, cut from their stems were the perfect size and fit.
Your are very lucky your working on a Slough car, as the French Cars Wiper Motor can only be worked at, by laying on your back & fiddling with the tips of your fingers. End result, writers cramp and sore back for days after.
Hope this helps, Davy