The extension is very simple:
The piece of MDF that forms the top is just something I had lying around and was about the right size. It sits in a rail made from a piece of softwood, also found lying around. The rail is made on the table-saw. I am so pleased I bought it!
The timber supports have a rebate at one end that sits in a matching rebate in the vertical part of the rail.
My idea was to make this something I could put in place when needed and store out of the way when not needed. The simplest solution to how and where to store it that I could come up with was to make a couple of brackets and screw them to the legs of the bench on the end where the extension will be used.
It's out of the way but easily accessible and right where it needs to be.
I might yet build a slightly longer (or should that be wider) knock down extension that attaches in the same way but has its own fold out legs to support the other end.
The challenge as ever is where to stop it if I make it.