#1 I would check to make sure you have the correct bulbs installed, did this just start happening, did it start after replacing the bulbs??? You're not supplying enough information.

 

I replaced the bulbs before its MOT and it passed so I’m assuming everything was ok.

Do you know what the correct bulbs are? Dual filament?

Also, there are no bulb sockets at the top part of the taillight…is that normal? Often wondered what that was for.

cheers

 

Since your third brake light works, It would appear your brake circuit works properly. I agree that it sounds like a bulb issue.

I would try replacing them before doing anything else. According to my local parts store, it is one of these