I then asked for help from the BSA Bantam Club and I didn't have to wait long before receiving the first reply! It seems the parts book illustration is wrong - the 'C'-shaped connecting link DOES NOT face forwards, but to the back of the bike as seen in the illustration opposite.
And I'm pleased to say it works! However... I think I shall have to weld some extensions to the feet of the centre stand as the back wheel still remains in contact with the ground, as does the front! Never mind, it keeps me off the street corners, doesn't it?