Just to conclude: I bought the Wadhams kit, but sent it back, because it seems to require that you remove much of the back of the light fitting.
So, I bought one of the Holdens replacement bulb holders.
This turned out to be a bit of a job because the chrome light fitting had corrosion issues - I have seen zinc alloy mouldings disintergrate in the past, but not automotive applications. Anyway, some holes in the chrome plating seems to result in holes in the zinc moulding - a couple fairly significant at around 10mm across.
I soldered the holes up and painted the non-visible areas with a silver paint - which I kind of hope will keep off the corrision issues in the future (visible chrome is all very good!?).
The other major issue is removing the old light fitting. Although this had disintegrated through corrosion, the bit attaching it to the chrome moulding had not. I managed to lever off some of the metal light fitting from the inside (light side) and then was able to push out the remainder. The pressed-in light fitting also holds an aluminium reflector (for the turn indicator) on the light side and an earthing ring on the back.
The replacement light fitting requires pressing/swaging-in. So, after putting in place the earthing ring and the reflector I expanded/swaged the light fitting with a large steel washer and a pair of vice grips. This worked fine, and provides an earth to the moulding, so the earth ring is not strictly necessary. The only problem I found was that I could not get the swaging really tight, so the light fitting turns if the bulb is twisted. Next time I take it off I'll probably solder the light fitting to the chrome moulding to prevent this happening.
For those wondering about solder repairs to holes in components, this is a fairly well established method of fixing small(ish) holes. I am not sure about UK cars, but cars in built in the USA, well into the late 1960s, often had lead filler in panel joints they wanted to conceal. I once found a door with a great big lump of lead in it as damage repair.
Lead is, of course, banned in these sort of uses today.