A special 'thank you' must also go to Andy Hennessy, in Ireland, for sending me the correct spring. As you can see it fits perfectly and wasn't too much trouble to fit either. No skinned knuckles to show for it too!
So, for anyone buying a new centre stand return spring, dodn't fall into the same trap as me: do NOT buy a spring from anyone who advertise it as fitting All Bantams, for D1 to D14 - as they don't! D7 upwards are longer, and won't come under enough tension to lift the stand.
YOU HAVE BEEN WARNED!