Some progress to report ... I got in touch with Hobkirks and they can order the screw (80p + 4.50 postage), but it will take a couple of weeks.
When talking to Stuart @ HObkirks in their technical dept he mentioned that I could still use the machine, if I removed the main Fetcher piece and the little knife. The cutter won't work but it didn't anyway!
I thought I couldn't use the machine until the Fetcher was screwed back so his information was priceless to me.
Am now sewing happily making up a Blue Dot Dover Jacket and I will NEVER, EVER undo a screw on the machine ever, ever again