Hi I was walking past a local petrol (gas) station yesterday when I noticed that one of their 5 foot fluorescents in the workshop was lit from one end to about halfway along the glow gradually fading along the tubes length. Several other tubes have recently stopped working in the same premises they are standard switch start types i have only ever seen this effect once before in a newly installed fitting in a shop in town.what's your opinion on this and has anyone else ever seen this effect elswhere.

Sounds like mercury migration. Where the mercury is pulled to the good end

That looks strange. I haven't seen the standard series choke ballast doing that, it usually starts to flash (the starter triggers), completely extinguish (if the starter cuts out) or glowing tube ends (welded starter contacts).
If the lamp is dimmer than others, I would rather guess for an emergency inverter feeding the tube with HF AC voltage not sufficient to ignite the lamp (runs out of battery power). A cause for that would be then some fault in the emergency lighting circuit (missing supply causes the emergency units to switch over to the inverter and as it lasts for too long, the batteries were just discharging to too low voltage).

It might be a contaminated new lamp. I've had this happen, although on HF. The new lamp started normally but one end gradually dimmed down. The good end banded up heavily within 15 minutes. After about 12 hours of steady use, everything cleared up, including the blackened end. I guess whatever contamination burned up. It's been normal ever since. The other lamp on the same ballast was not affected (although the ballast is wired in parallel).

If the ballast outputs feature DC blocking capacitor for each lamp (that was not the case just with early ballast designs), then they can not interfere with each other - even when one lamp rectifies, it does not cause any DC current, just DC voltage component across the faulty lamp. And no DC current means no electrolysis effects (mercury migration,...) could happen.

But indeed, with new lamps it could be either contamination, or nor properly fired amalgam capsule on one side. But that will get sorted after some hour by itself.

But even the emergency thing would "get sorted" after some time by itself: The lamp will just shut down completely (or the emergency units will switch it back onto the normal ballast due to the flat batteries - depend, how that is designed)

Does it look like this?

Yes that's how it looks medved I think it must be just a poor tube causing it the wiring on the building is around 55 60 years old and I don't think emergency lighting existed then. The fittings were replaced because the original fittings had rusted out

It could be lamp rectification.