My extra spring is a long tension spring, one end looped over the accelerator cross shaft arm where it meets the rod that comes down from the ball joint on the carb, and the lower end attaches to a U shaped bit of wire coathanger with a small loop on top of one of the uprights. The U is shaped to hook around the underside of the chassis rail.

Nothing permanently attached or drilled, takes far less time to do than describe, and passes scrutineering for "additional throttle return spring"

Cheers

Simon

Location: On a hill in Wiltshire

Hi
In the absence of suitable elastic, I have just found and fitted a spring as per your suggestion, Simon. On a test run round the local area it is now possible to keep an even pressure on the accelerator pedal. Thanks to all for the suggestions.
Regards
Colin