I have to change this capacitor on my water pump. thought it would be a no brainer, but there is nothing on the capacitor to indicate to me which lead goes to which wire (which is live and which is neutral) Its just a black box with two black wires coming out.

The old capacitor is still in situ. But the replacement part is genuine, whereas the current (fried) one is generic and they are shaped slightly differently. So no clues there.

Is it possible to hook this up wrong (backwards, or does it not matter?

Nope, no issue, it goes either way round

I thought that Capacitors have positive & negative leads....

I thought that Capacitors have positive & negative leads....

Only if electrolytic or Tantalum.

The run capacitor in a single-phase induction motor is an X type, metallized film, self healing, device specifically designed to be used in AC mains applications.

All fixed, so I can have a shower. My wife thanks you Crossy.

Thank you very much

Glad to be of service