Probably you use an anti-cycle ignitor that prevents cycling completely by stabilizing the lamp when it wants to go out? I've seen a GE Lucalox 250W XO lamp cycling in bluish mercury once.

It's still quite stable, seems to run better the longer it runs!, the ignitor on this is Thorns G53455, it hasn't got anti-cycling as far as I know?

There were Thorn Super APU and Atlas APU anti-cycle ignitors that made by Sabir, France and they have model numbers similar to yours. Several of them, monitoring lamp conditions and stabilizing the lamp if it about to extinguish. I can't see the detail of your ignitor, because of the viewing angle in your picture.

That is quite a high arc voltage!

How exactly does an ignitor "stabilise" a cycling lamp??

The ignitor can stabilize a cycling lamp by firing a pulse at each of the mains half-cycle before the lamp extinguish. See here .

I thought SON cycled because of unusually high arc current though?

Try to put a cycling SON lamp with a Tridonic ZRM 4.5 ES/CT ignitor and see whats I means. Cycling lamps on this ignitor would just flicker.
And SON lamps cycling because of a high arc voltage.

I'll try that with the 150 E I have, that lamps worse than this one!

This is something that i brought up in a discussion with Dor years ago. Ignitors typically have a SIDAC or a firing circuit for a thyristor which triggers when the capacitor in there is at 200V or so. The reignition peaks at zero crossing of aging lamps will eventually get the capacitor there, and the ignitor will start buzzing with the lamp still lit. Assuming that a cycling lamp would tend to go out by failing to reignite after a zero crossing, the ignitor may keep a lamp going for a while which would already extinguish without this "help". When the lamp is already fully going out and immediately restarted by the ignitor this looks like a lamp that is dimming/flickering repeatedly in a cycle of every few seconds