Yesssss recognise this one didn't we have it on 400W merc ballast in the end?

Yeah, James said it would match the lamp current better on 400watt SON.

I have one of these I've never lit tho

Put it on a 400watt SON ballast, it will take a while to run up, but when it has, get a dark glass, (like a welders mask) or the same, and look at the arc, its like a little green candle flame , really weird to watch.

James loves firing up these old weird kinds of lamps that I wouldn't dare try lol

I don`t know, we dared try the 1200watt Brite Arc, that radiated us all out of the room!

Most of the special lamps are robust things, I am sure even if we ran this on a 1kW SON ballast it wouldn't have suffered - just look how huge the electrodes are and how thick the quartz is on this model! When things are made in small quantities the business isn't big enough to optimise the design for lowest production costs, so they tend to be greatly over-engineered.