Popped into my local car accessories place at the weekend to get some cleaning goodies, and they had some 1/2" impact-driver sockets in both 41mm and 46mm for 7.99 each.
They're made by Draper and fit perfectly as there's not much shoulder on them.
You should be able to order them through your local Draper supplier. Details here:
http://www.draper.co.uk/ (note that these aren't listed in their online catalogue for some reason! )