I went to our local B and Q today which for those in USA or Europe who mite not know is a DIY and homeware shop anyway last time I was there it was lit with 4 tube flourescents lamps which mite have been on HF gear but I'm not sure this was only a year ago between then and now they've changed to LED (sorry to swear) and theye mostly were only glowing dimly now I'm not sure if they are dimmed during daylight hours or are just starting to fail either way they look terrible.

Dimming of LEDs is not that fast. If it was failure, most likely a few would be dimmed and the others normal

Possible explanations for the dimming :

 - Indeed some dimming control

 - LEDs of way too low power rating used in the 1st place (and working at max power). This is quite often the case, where LED products are chosen based on an energy saving goal stated by beancounters ("you must save 50% energy") - which in turn determines the power of the new lighting and the light output, or based the "equivalent to" ratings that come with the LEDs themselfes, which are usually fraudulent by a factor of ~2. Either way, the wanted savings at the same light level are not possible as the efficiency difference between the LEDs and the other lighting system is not what it was claimed to be (by magnitude and sometimes by sign too), so the light levels are dropped

That makes sense its one of those places where the beancounters are very much in charge there are a few lights on àt nearer normal brightness  but they are all in a separate area

Or maybe the LED array has bad heat sinking, And the ballast is lowering the power because of high temps.

Could be either reason whatever it is I don't suppose il ever find out! I will probably go back there in six months and see how the lights are doing

With the thermal foldback heat management I doubt the power would get reduced lower than about 30..50% of the full rating, even in most adverse condition. And that is just "slightly dimmer"; to see the difference, you would have to have side-by-side compare, so I doubt you would ever notice that in action.
For defective units I doubt it would be the majority of them.
So what remains is the active light control. With many installations, the dimming is usually limited to some minimum level, just to make visible, which fixtures are turned On (and just dimmed by the light management) and which are OFF or not working. So with sunny day you will see them just glow "uselessly" dim.
With fluorescents such minimum level was usually about 20% or so (dimming below does not save that much more energy, given the ballast losses and necessary filament power) and that is still a level appearing as "full power" (when there will be a lot of sunlight and you will have no reference to compare).
With LED's the minimum limit (diming below does not mean any significant energy savings; with a 20W LED that means 0.2W) is around 1% or so. And that is then quite noticeable "dimmed", so may look unnatural...

It was a sunny day when I was there so it seems very likely that some sort of brightness reduction happening the lights that were at normal full brightness were in a part of the store with no natural lite il try and go there on a dull day and see if I can see any difference

i would not expect thermal protection dimming to be uniform on all lanterns

There are allways hotter and cooler lanterns, from being near or far from air conditioning vents and so on. If the dimming is relative to the temperature, they would be in different brightness. If the dimming is one level, i would expect to see some lanterns cycling between the "low" and "high" setting