Thank you

And particularly for the suggestions, aussiegal (I feel weird addressing you like that, as an expat in an area with not very many Australians... that's me!). I tend to just eat within a range (and I don't consciously plan it as such- more "that's low calorie, that should work out", and then eat what I want), but as it's a 500 calorie range I probably get some variation already in my diet. I think I'm going to be going to classes or similar when my gym membership expires (I'll be joining the gym when I get back from visiting my folks, but I'm not sure the numbers add up for the time I'll be away)- although in terms of intesity, my workouts tend to be long and intense (as in, much more intense and I start feeling like I want to vomit). (Side note, I have moderate DOMS- I can walk, but it's painful- today. I think upping the weights on my hamstring exercises is just what I needed) I tend to be a little bit conservative on changing things (I look at "cheat days", for example, and go "what are you, nuts? That's way too many calories, you'll blow your hard work!")- but if the plateau persists I'm going to talk to my doctor about it (who hopefully won't say "you have a healthy BMI stop stressing"... BMI isn't the be all and end all- I know neither is this number I have in my head, but at least that's based on something about me).
The thing that really upset me yesterday is that I couldn't think of any reason why I would be gaining weight. As you say, I'm doing everything right. My calories are low, my nutrition is good, my sodium is a little high but not critically so and I can't see it'd account for that kind of gain, and I exercise. I've always hated being punished for doing the right thing, and that's how I felt yesterday.
What I was trying to do was, rather than the milk, switch it up for (low fat) natural yoghurt. Which tasted good but was higher calorie for less protein punch. I still put a little natural yoghurt on my porridge, but almost a garnish now, rather than the whole thing. 300ml of milk adds about 100 calories, so it's not huge. (My breakfast was 284 calories)