Thanks for the comments, smartarse ones as well
Had a good laugh.
The seat does not unscrew thank goodness, or I would have landed on the floor many a time.
Sometimes I need a good kick up the kasaust pipe to make me think again, and Niko did just that!
For some unknown reason I rejected his idea when I first thought of making it higher, without even looking to see how easy/difficult is was to do as he suggested.
I've just unscrewed one of the castors, and they are threaded M10, so suitable spacers of large diameter will be easy.
Len,
I only use it at the Workbench, never when using the machines. Your mishap made me wince.
Geoff – Thank you Niko!