Any electricians in the room?

[fluked]

I have double 2-way switches controlling the lights on my hall and landing, so either light can be switched on/off at the top and bottom of the stairs. Recently, the upstairs light stopped working. When we tried changing the bayonet bulb, it flashed on momentarily as we started fitting it, then went dead.

The downstairs light is working normally..However, when the light is on and I test its switch with a VoltAlert 1AC-E II device, it gives intermittent beeps about 1 second apart, rather than the continual beep I get for other working light switches. The VoltAlert instructions don't explain what this means. It only happens for one of the two downstairs light switches - the other doesn't beep at all, even when switched on.

Neither of switches for the dead upstairs light make the VoltAlert beep, no matter which way I switch them.

Can anything be concluded from these observations? Is there anything I should try before phoning an electrician?

Disclaimer: Not an electrician.
I would trip the breaker for the lighting circuit, confirm it is off, before opening up switch panels and check nothing has come adrift wiring wise, as that's compatible with what you're describing, though not necessarily the issue.
Failing that, give both sets of switches some rapid on / off action, to ensure not just a switch gunk/grit build up - less likely.

Maybe try and see if you can raise PigletJohn (sp?) on Household/ DIY forum. Don't think he's an electrician as such, but is knowledgeable and practical.