I'll say this as it did actually happen last week. Friend who is new to Land Rovers and only recently got a Defender had to replace the light switch. It was dodgy and needed TLC to come on. Replaced and main beam wouldn't work but flash to high beam would...
Guessed the problem yet?
I called around, looked at what he had done and ... turn the ignition on... yep main lights worked 
Now I'm sure this isn't the issue here, but I thought I'd just post the bleeding obvious.
Now on to the rest of the possibilities...
Have you checked the fuses and state of the fuse panel - dodgy contact?
Is there anything (e.g. Turbo timer) wired in to the ignition or light switch?
MY15 Discovery 4 SE SDV6
Past: 97 D1 Tdi, 03 D2a Td5, 08 Kimberley Kamper, 08 Defender 110 TDCi, 99 Defender 110 300Tdi[/SIZE]
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