Oops, I forgot to update this story.
Andrew came over, checked all the parts and confirmed that it was definitely a broken hand controller.
We looked up the part number and found the manufacturer in Hong Kong. Emails back and forth and all is sorted.
I ordered a new Hand Controller - $17 and all is well
Thank you to David and Andrew M110 for your helpful suggestions and to Andrew (Astroman) for his hands-on help.