Can anyone give me any tips on how to locate a short in my house lighting
circuit?

I was sitting here at my desk and the whole upstairs lighting went out. The
circuit breaker had flipped. When I tried to un-flip it, I saw a spark and
it wouldn't flip back, so I conclude the short is still present.

No alterations have been made to the wiring for several weeks here, so I
can't imagine what might have happened. I don't have rats in the loft.

I have a mains testing screwdriver at hand (and a multi-meter if needed).

Thank you!

Al