Thank you all - especially "Redneck" for the link to the Czech manufacturer - I have sent them an email with the details
If that fails, yes, a new chuck is the only viable solution.
I know others are correct - it will be here somewhere. Maybe if I demolish the shed, the key will materialize in the rubble. The good news is that this incident has already caused a major clean-up with several trailer loads gone to the tip. This is contrary to my usual practice of never throwing away anything that "might come in useful" Amazing how some of those old bits get used when nothing else would do the job. Actually, my dog Jenna or maybe one of the tame possums are the most likely culprits. I don't mind them borrowing tools, if only they would put them back where they found them
Cheers
Alan