Our PA28 has developed a problem. The Low Voltage indicator keeps
coming on. The engineer has replaced the Voltage Regulator which didn't
solve the problem at all. Then he changed the Overvolt relay. I flew it
for 50 minutes with no problems and then the next time I took it up
(some days later) the light came on within 10 minutes of takeoff, then
7 more times in the 1 hour flight.

It comes on at seemingly random times and can be reset by recycling the
alternator switch. Sometimes it stays off for 10 minutes, sometimes
half an hour (which makes tracking down the problem even harder!)

Any ideas?

Your alternator is going offline, the battery is powering the main bus, and
thus the low voltage light is on. Focus on why the alternator would go
offline. Verify a good connection on the alternator field.