I was told off for using Jubilee clips on the A ( on the L/H fork leg as I fitted the front brake cable correctly ! ), so I set to and made a new stop using Amal handlebar clamps - looks more period I suppose !!I now use a cheap "jubilee" clamp on the rhs. Not as elegant as your fix but it does the job. Trev, I followed your instructions but a 3rd hand is still useful.
According to wikipedia, which I always believe, the jubilee clip was in production before WW2, surely making it a suitable period fitting for later A singles and all A twins.
http://en.wikipedia.org/wiki/Jubilee_Clip