my diesl octy has developed a slightly annoying habit of turning the dip beam off when driving.

the front fogs, main beam and rear lights stay on, only the dip beam and the dip beam warning light on the IP go out.

any ideas?

Headlight switch or relay would be the prime suspects.

switch... I had similar on my old car. And not long after my main beam was dodgy, had to replace the stalk.