If the fitting struggles to start the tube, might be worth replacing the series capacitor.....

I would have thought it would be the starter.

On a fitting with a SERIES cap, a weak cap will make striking the tube virtually impossible. A good example being an 8' 125w circuit with series 7.2mfd cap - older fittings often struggle to start the tube, stick a new cap in a d bang, good as new!

I'm hoping the capacitor will be OK. The fitting doesn't look as though it's had much use. Anyway, it's impossible to come by 3.85uF +/-5% 440V capacitors these days. Any replacement would probably have to be a hand-picked series combination of two higher values. Once running it's perfectly fine, not dim or anything. I did have a 6' fitting with weak capacitor. That started no problem but it was noticeably dim due to the low capacitance. I have heard that 50W switchstart had low preheat current and starting issues, which is why they standardised on SRS.

Hmm, no, you might get lucky with a 4mfd cap, which I';d guess would be close enough.....

It's been running really well on a variety of tubes. It's just the starts that are a bit protracted at times.

Ah well, could be worse then I guess!

It seems to be starting pretty well now. The last tube I've got on test (Atlas Pink) starts pretty quickly. I wonder if the NOS starter has settled down? The lamps are flickering much less during running, so perhaps any rogue glow discharge in the starter has subsided?

You have a 50w tube IN PINK!!! Never would have assumed such a thing existed