I now have a collection of three of these valves but they all seem very rusty and I'm not sure about re-using any of them.
Does anyone know of a refurbishment service from a manufacturer or even where I can get some NOS please?
They are readily available new from most suppliers. There are extremely few repairers that will match new quality for almost the same price. So I wouldn't even look at used.
Well, I've looked up the part number (599443) and they are in stock at various places. Dunsfold have them at £134 which is not exactly "cheap as chips"!