1000V capacitors

[Mike Watterson]

Is that a genuine increase in value or an artefact of measuring capacitance by timing how long it takes to charge? The "Peak" and Chinese testers all use this method which will show a leaky cap as having increased in value.

Um, even simply measuring voltage drop at RF gives an apparent increase in capacitance with a leaky cap.
I use the electronics of a flash on a "disposable" camera with the 300uF cap replaced with 1uF, 2 x 1M in series as one would have voltage rating exceeded. Remove xenon tube.
The "earthy" end of the cap connects via a neon with 0.1uF. A solid "on" is a fail. A good cap flashes once and neon goes out. You can use it as a 100s of M Ohm meter by timing the flashes.
Measure PIV of diodes or transistor Vce if you bypass neon and use a 20 MOhm or higher DMM (or a VVM), non-destructive due to low current.
I've a 2nd higher voltage one from a standalone flashgun that has a uA meter in the case. It uses 4 x AA. The modern ones made from guts of "one time" film cameras use a single AA. An old one uses a PP3 and sprung on switch.

Higher value low leakage caps are rather dangerous when charged to 600V.