It's a 150W MH running off a simple reactor ballast.
I'd already tested the ballast by replacing the bulb with a 300W linear halogen which lights at about 70% brightness. I just didn't have any spare MH bulbs and my current one is brand new and unused (doesn't necessarily mean it's good though). As a new ignitor was a cheaper option, I've gone for that rather than buying a 2nd new bulb.
Only catch is that it's going to be a tight fit getting my replacement 600W HPS digital ignitor into the luminaire - it should certainly be strong enough.