High Level Brake Light linked with main lights????

Hi,

I got stopped by (luckily) a friendly policeman yesterday to tell me my high level brake light was permanently on. I've realised that it seems to be linked to the main lights, if i turn them off, it goes off! Ant ideas what may have caused this change, it must've happened in the last few months as it passed the MOT (uk) in August.

Thanks

woo