Turns out 4µF is not the ticket and made the pump quite noisy, referred to the last 2 rads in the system where you could feel them vibrating as well as hearing it, which was handy being bedrooms and the front room.Thanks. I went digging on google and found a few calculators (that agree with each other) for 1ph motor run capacitance and it looks like the 2µF capacitor is intended for the lowest power rating, 40W (it's a 3 speed pump). At 95W on 3, it suggests a ~5µF capacitor so there must be a fair tolerance on the acceptable capacitance. I'll tag getting a replacement on the bottom of the to do list, just to get the cover back on if nothing else...
2off 4µF caps in series for testing and it's back quiet again. I can't find a cap online for less than about £7+postage so I guess the £8-9 ones on ebay might not be the rip off I initially assumed...







