These cpus suffer from crappy thermal paste Intel has used in them, they must be delidded.

AberTim wrote on Today, 11:16:

It is my "assumption" that the 'k' CPUs have all been damaged by Overclocking.

Yes, that's possible. I was overclocking the 3770K ES and trying to get it stable at 4.6-4.7 GHz, but it seems I overdid the core voltage.
After that, it won't overclock above 4.3 GHz.

Desktop Ivy-Bridge has a regular paste under IHS instead of solder. These CPUs are almost 14 years old.

Thanks guys,

I did get the impression that the temperature of the spreader lid wasn't consistent with the temperature of the core, but also thought the die to spreader connection was of the "hard" glue bond. Now I know different, I've ordered a de-lidder from AliExpress. ("delidders" who'd have guessed?)

Also something I didn't mention about the CPU's, was that all three had patches of discolouration on the underside, which although did buff off with an oily rag, they very much struck me as heat generated and were around groups of 4 pads. My guess is that the motherboard current sense circuitry may well have saved the CPUs from complete destruction. due to thermal runaway. I must have around a 100 CPUs, but have never seen this discolouration before. That said I've not "knowingly" had such an easily over-clocked type either!

The last actual over-clocking I did was a 80386 DX-33 around 1991. I killed it, and it cost me about 2 weeks wages, and I sort of lost interest in it after that.

Pretty sure 386 can't be killed by any common kind of overclocking. There's no voltage adjustments and it's feeding directly from +5V line from PSU.
Modern CPUs? Yeah, those are very easy to kill, they already run on the edge of possible from the factory (cough cough 14900K).

Ivy Bridge is somewhere in the middle, but statistically it's very uncommon to encounter a degraded from overclocking CPU, not a lot of people bother with that, especially on midrange stuff.

AberTim wrote on Today, 11:16:

My question is, does anyone "know for sure" if the 'k' versions (when new) did have the same low
power consumption as the plain i5-3570?

I am not prepared to confirm this from personal experience, as I am not involved in ‘high-performance sport’.
However, it is traditionally believed that CPUs with "high leakage currents" (i.e. with higher consumption at idle) achieve more highest frequencies in extreme overclocking.