Thank you for such a helpful video

If it's flicking on/off, it's often the switching chip reboot looping because its own little power supply is failing (small capacitor).

100wt bulb driver giving out low voltage , can capacitor cause it

I must admit I thought that was the only electrolytic cap on the line side of the regulator (i.e. after the rectifier and common mode filter inductor) and so I thought it would have 340v across it and, given that the replacement was rated at 50v, I rather expected it to go bang.

Simple, measure the cap with a real LCR meter at 120 Hz, note the Dissipation factor stated as tan delta. Look up the datasheet, there they state the new DF and the "bad" value at 120 Hz. This is the only good way. ESR is not stated in the datasheets because DF is used for decenia and does a better job. ESR at 100 kHz is sometimes used in datasheet for SMD caps. In most datasheet nowadays they also state the impedance at 100 kHz. For bigger caps the reactance is very low so the impedance is for most made by the ESR. However, ESR is frequency depending and at 100 kHz big caps can be inductive instead (look up SRF, self resonance frequency) On average, a very rough estimation, the ESR is at its lowest value around 30 - 40 kHz but can vary wild because of the many sizes. DF is the ratio between the reactance (capacitance) and resistance(ESR). Together they are the impedance. So knowing DF and Cs you can calculate ESR (there is also Cp but that gives DF bases on the parallel resistance and needs to be converted first if you somehow think you need ESR instead of DF. The nice thing is there are tabels from the manufactures but those from good brands are very close. I looked up some good brand data sheets an it turns out they are almost equal so I use a little table I made from comparing a few brands and if the measured value is close to the new value it is OK, if it is higher and critical, or a not so common can size, I look up the real datasheet. Those testers are like a DMM with capacitance function, only suited to measure good caps. They can completely go wrong on bad caps. An ESR meter is a handy first indicator to quickly see if a cap is dead or good measuring in situ. Good is not even a guaranty, if there are good caps in parallel with a bad one it will show a very low ESR, and also inductance in the circuit can mess things up. This is only to pinpoint very fast suspected bad caps, desolder them and then test them with a LCR meter, on C and DF, and in some cases test them for DC leakage. Then you will really know if it was bad. Instead of the ESR meter, if the circuit powers up, you can watch the ripple voltage with a scope over a cap to get an indication. If the ripple is fine, the cap does what it needs to do and if it stays cool the ESR won't be to bad either. In doubt desolder it, let it cool down (if you have bad desolder gear, heat influences the ESR / DF) and use a good LCR meter.

Rubbing his hands all over the caps like some kind of MAD LAD.

Wath happened with your shugon t26