I almost don't dare write this but things are looking up. After a weird breakout of nodules on my chin last week, it has all cleared up. My left cheek has stayed pretty clear, and my right cheek is a lot better. I am still getting tiny pimples on my rights cheek (I may be cursed with these for the rest of my life) but they don't bother me so much because they are not deep, under-the-skin ones. They disappear in a couple of days with the differin. The scarring is annoying because it looks like acne, but this will fade eventually.
I have had no milk or yoghurt for over three weeks, but must admit that I've had a bit of cheese this last week with no disastrous side-effects. I have also been taking my multivitamins regularly and trying to eat more vegetables. This has hardly been a scientific experiment and I don't think I can necessarily attribute the clearing to the no-milk diet. But having read about the things that go into milk I don't think it will do me any harm to cut down on dairy. I used to drink at least of glass of milk every day and eat loads of cheese, so will probably be healthier in the long run anyway.
I am a bit suspicious that things could get worse again - I have PCOS so my hormone levels are really unpredictable and I don't have any regular cycle like 'normal' women. This means things can turn a corner pretty quickly, but I'm trying to keep my chin up!