Hi, this is an Audi A3 question but there is a lot of excellent knowledge on this forum. 
With no lights on the brake lights work properly. 
When the side or headlights are turned on the high level brake light operates normally but the brake lights located in the rear clusters stay on all the time until the lights are switched off.
So in other words, the brake lights work at the same time as the rear lights. 
 

Any ideas of what to check?

Edited 5 August, 20223 yr by prt57

8 minutes ago, prt57 said:

Any ideas of what to check?

Rear cluster earths, front sidelight earths, dodgy brakelight bulb if 5/21W

Check you've got the right bulbs in all the lights and they're in the right way round. 

Ok, thank you very much for your thoughts so far.