That is actually a very good point Malc
It is true that the set will be unlikely to be switched on from morning until night everyday of the week.
I did download the full data sheet via the link kindly supplied by Catho, not sure I can fully take in all of the information contained within and also me being a bit thick couldn't translate the dT/dV spec. into anything like a simple yes/no it is/is not suitable.
Anyway more to the point I had a look at the circuit diagram, and the Cap C518 is indeed the line flyback cap, well the original value 0.028 μF seems a highish value compared to more modern sets where you would more usually see a max of maybe 15nF more usually about the 5 to 10 nF range also rated at 1400-1600V range as there is usually a 1.2kV 11μS duration flyback pulse, but I digress.
Edit: The way I was actually taught with general rules of thumb (and bodgery

) in the line flyback dept. was the replacement cap should be physically similar in size (this discussion), have at least the same or better voltage rating eg a 1.6kV cap can replace a 1.2kV but not vice versa, it was ok to use the next value up but not down.
That last bit was explained as follows, if you reduce the capacitance value the flyback pulse gets narrower but higher in peak voltage, if you increase the value the pulse gets wider but lower peak voltage, but this is also a trade off, if you increase the width/lower peak you also keep the output transistor switched on in it's transition period for longer (slowed the pulse down) and so it dissipates more heat.
This is sort of between a rock and a hard place, ie. you reduce the value too far and you risk high EHT and problems with punch through on the lop transistor, transformer and other related components, increase it too far and risk the lop transistor running too hot and failing, both conditions are not good for reliable long term repairs.
Ps. similar general rules of thumb apply to the snubber caps in Switched Mode PSUs.
see, I am a mine of useless information lol
