Approx 5 years ago they certainly could as long as they hadn't put your rent up within the 6 months before. I had an agent firstly put it up just after renewal time. I contacted the tribunal who said they could as long as it hadn't gone up in 6 months before. Which i found on the Internet then too.
Then they tried to put it up after having to spend a fair bit on the house two months after. That time I had to get the law pointed out to them as they seemed to think they could change it whenever they liked.
That's great news that the law has caught up with common sense. It did always seem crazy to me that you have a contract with xx dollars on and that could just be changed when they felt like it, but you'd struggle to move out if you didn't like it.