Rents haven't risen as much as house purchase prices over the last few years.
In 1999 one of my student houses cost £75000 to buy and each student paid £50 per week rent. Now a similar house costs at least £225000 and my students pay £80 per week each.
In 1998 I bought a 2 bedroom flat for £35000 and charged £400 per month rent for it. My son currently pays £600 per month to rent a 2 bedroom flat which cost his landlord £160000.
Small annual increases are far better than massive knee-jerk rises. Having said that I actually don't increase the rent if any of my households choose to stay for a second year.