My grateful thanks to both Bob and Roger for their help. Also Andy Burnett, who searched in vain; at least for the five, that were required!
Vintage Restorations, were indeed able to help in the end and can also supply the springs that retain the bulb holders in place.
Check out https://www.mg-cars.org.uk/vr/ if you need these bits.
I thought that I had checked old MG parts lists, just in case, so there must be a model, I’d missed, that had a similar setup to a Slough Traction!!
https://www.dropbox.com/sh/oeac46dcc29b8kt/AADD7KwkNFrVVcWz69_0AeB0a?dl=0
Martin N
Martin Nicholson