I took the car for a spin this morning and perhaps there is an improvement but I think I have evolved a technique of sliding the side of my shoe against the side of the bonnet to act as a 'friction damper'. It is possible the spring is still not strong enough.
I fitted springs to each carburettor so they would both shut in the event of linkage failure. The springs pull down on the levers on the spindle and are attached to bent wires trapped behind a suitable water jacket bolt. These have a ring formed at one end and a hook at the other and I think they came from a superior galvanised coat hanger!