I had this 'intermittent' problem. I had the three lights on and no brake lights.Abs light comes on with ignition as it should.
Brake lights are not working now so must be switch, Sod's law these were working when I bought it.
It must be intermittent that makes sense as the 2 dash lights were not on when I drove it home.
It was the brake light switch. Think it was about £35.00 to replace and two minutes work to fit (although it's awkward to get at). Do a search on the site and you'll see some pic's regarding the newer version.