Thanks to Rod Renshaw for the description (pp26-8) of his implementation of Jim Whetren's idea for an easy-to-use clamp.
Thinking about my own small Astra drill and re-reading Jim Whetren's description led me to a slightly simplified version:
The vice is free to slide fore and aft and is instantly clamped by the knob under the table:
In my version the clamping pressure is applied by the large washer and I have adopted Rod's suggestion of knurled top hat nuts.