Alright , why are tubes seemingly so dim on damp cold days ?

Phils description is not quite correct. There is no heater in the starter bulb. There are a pair of contacts with one or bothh made from bimetallic strip. The contacts are open when de-energised. The enevelope of the starter contains a gas mixture, normally a neon xenon mix and freguently a small amount of radioactive material. The circuit is supply live, ballast inductor (choke), fillament at one end of tube, starter, fillament at other end of tube to supply neutral. Note that the gas inside the tube is effectively in parallel with the starter. At switch on the gas in the tube is not ionised so is non- conducting. The full mains voltge appears across the starter contacts and the gas ionises and becomes a low resistance. The current is limited to the power rating of the full tube e.g. 80W this flows through the filaments and the starter, heating all three. The bimetalic strip get hot and the contacts close. This causes more current to flow through the filament but the starter gas cools and the contacts open. This causes a sudden drop in current and th induced "back EMF" from the ballast inductor appears across the (pre-warmed by filaments) gas in the tube. The gas ionises conducts and glows emitting UV and short-circuiting the starter which now does nothing.
To the original question there are two "wear-out" failure modes, the contacts errode due to either normal use or more likely allowing it to do multiple re-starts of a faulty tube. A capacitor failure open circuit will cause faster contact wear, The other wear -out is poisioning of the gas fill due to out-gassing of the starter components. There are of course true failure modes like leaks or failed capacitors but these are random. There is a odd old stock wear-out for some of the starters with radioactive gas. The activity decay with a half life dependent on the isotope used but for some sitting on the shelf for years will cause failure due to the isotope decay.
It's good pracicw to replace the starter when the tube is changed.
While the LED replacement tubes hav become popular there are concerns over their safety see:

https://www.electricalsafetyfirst.org.uk/professional-resources/product-safety-unit/led-replacement-bulbs/

and

https://www.electricalsafetyfirst.org.uk/media/1206/best-practice-guide-9.pdf

for best practice guide.

Rert G8RPI.

Slight digression from the OP.

The link from Robert on the safety of LED tubes is dated 2011. Since then they have become more popular and there are plenty on the market that are safe. If you go for those imported illegally with no CE mark then you may have a problem, not that a CE mark is a failsafe guarantee on quality.

Also the safety aspect is about replacing them. Using basic common sense when doing this would help to eliminate the risk of an electrical shock.

Well, I seem to recall way back in the early days of Fluorescent light fittings when we used to put the choke between the floor and the ceiling, there was a starter switch similar if not the same as Phil described. With some there was a normal wall switch plus a push button switch, the client flicked the switch a couple of times, crude but it worked. I think some of the square shaped choke were actually for small Mercury Vapour lamps. These chokes were filled with Pitch and when faulty the Pitch would melt and ooze out and make a real mess on the ceiling. How things have changed.Apparently our "special partners" across the pond used Bi pin end cap from day one, whereas due to supplies, we were stuck with Bayonet cap. This of course was all during WW11 when extra lighting was required for munition work. Out of interest, ATLAS brought out a transformer switching arrangement for dimming fluorescent's and BR had them fitted in signal boxes. Rert G8RPI offers a good comprehensive explanation above. John

Having been "on the tools" and tech college qualified for nearly fifty years, you tend to learn a bit, the early ones used resistive heaters which could be clearly seen through the glass and I believe were "normally closed", later ones used the ionizing of the gas to heat the bimetal, and were "normally open" the later ones were much better,, and electronic ones (at a price) have made a brief appearance, but all that aside what am I going to do with my Twindustrial 8 foots, now that tubes are about £16 each, and by all reports the 8foot replacement LED tubes a) bend in the middle when they get warm, and fall out b) give so little light as to require two in each fitting|? seems to me that at the moment the cheapest way to go is to fit two four ft single end fed LED tubes to each unit, and rig some form of support at the non feed end, (the centre of the fitting) Anyone with second hand but working 8ft tubes in east yorkshire, I could be a gratefull customer!