Hi all, Got a problem started yesterday. When I switch on headlights only sidelights come on. If I then switch to main beam the headlights then come on but no main beam

. Anyone got any ideas? I've checked fuzes, bulbs etc and can find no problems there. I'm guessing it's a loose wire somewhere but can't find anything. I've had one of those headlight boosters fitted a few months now. Wonder if that has anything to do with it.