To be honest, I didn't get around to doing this. I had new tyres fitted and it did seem a bit better afterwards so don't know if the old ones were out of balance. But the problem was still there. However, recently it seems to have improved. I've been on some long journeys and do, if it's safe, try and brake hard once in a while as I read somewhere that some deposits can build up so it can help clean up the surfaces. Might be worth giving that a go one evening or early bank holiday morning.
I find it hard to believe that cleaning up that surface is going to make a difference, especially given how the symptoms for me aren't always consistent with speed/braking power.