I know this is true for dual voltage motors but this true true single voltage motors? Stu is in the UK where 120 volts is not an option. I have a single voltage motor rated for 208 volts and ir has a 220 volt a.c.,145 mf capacitor. I would replace with a 240 volt capacitor even if it meant a lower capacitance. It may start slower but it will start. Once running, the start winding drops out so there is no effect on operation.
It's hard to say without measuring the voltage on the cap during starting- most US motors and many UK motors have dual run windings even if
they are labeled as 220 volt only; the start leg is across only one of the two series run coils so the cap only "sees" 110V.
Stu if you can fit a 250 volt cap in there I would do it but my guess is it won't physically fit and you'll need to mount it differently
You could do a test with your voltmeter to know for sure- you can take a reading even with no cap installed
-M