Felt the "urge" to go to the other ReStore today, and by my luck I found a Fulham Workhorse 5 buried under a mass of cables that called out to me.
Tested it and my 55w lamp, and it was beautiful. However, the current connecting situation is unsafe.

Wondering if anyone here may have a socket to spare before I go to E-Ripoff.

Pic of course:

This is the least expensive BY22D you're going to find...
https://www.ebay.com/itm/144423140859

The difference between a normal bayonet and a by22d is that little isolation bar between the contacts.

If you happen to have a SOX 18 running in a proper by22d socket, you could consider swapping that one out for a conventional bayonet socket, and using the proper one for the 55w tube. The SOX 18 ignition voltage is so low, that it's highly unlikely that you'll ever have a flashover in a normal socket. The 55w one will have a somewhat higher ignition voltage.

The main risk is when the bulb's gone open circuit i guess.

This is the least expensive BY22D you're going to find...
https://www.ebay.com/itm/144423140859

Yeah that's the one I had in my cart, well decided to spring on it.
Thanks

Okay, suppose it's time to mark this as closed.
The socket arrived, wired it up and now I can finally make sure the base is bottom up!

From my experience, I have had no problems using normal British standard B22 lampholders for SOX low pressure sodium lamps.