Thanks for the help guys. It did turn out to be my brakes- but I had the wrong wheel. I could have sworn it was from the front, but I got my mate to stand outside while I drove past slowly, he said it was definately from the back. Lesson learned!

brakes usually squeal when the pads are worn and the disc is concentric meaning they might need machining but if it only does it when you brake i would say it would be the brakes

Check your brakes again. Instead just looking at brake material look for an indicator, a little metal clip. Look for witness marks along the rotor and check that the metal brake cover is not touching the rotor.

My daily driver, a Honda Accord has developed a problem where it has a grinding, squeeling coming from the wheel. I had a look the other day and the CV boot was all torn up and grease was flung everywhere. Easy peasy I thought, replaced the CV and cleaned and repacked the inner joint while I was there. The noise is still there. I'm guessing bearing but its $260 for a new one, one side only (!). So before I go spending the money I was hoping someone else would have an idea, there being a lot of people in the know here.
The noise is intermittent, only comes on sometimes. It usually comes on with the application of the brakes, and will stay on when I release them but only a bit quieter, so now I suspect the brakes. I checked the pads and they're all good, plenty of meat. Anything else I could check?