If you feel your prices reflect this year's going price then only increase by a pound. However, if not then increase by £2.
I learnt a short while ago, that I will always increase my prices every two years to get my customers used to it. The increase can be 50p or a pound as the vast majority of my prices are really good now, for instance I picked up a new house in an area I'm expanding into @£27 a clean. Granted, this may or may not sou d good to some, pending your location but many in my area don't feel you can charge good prices. Clearly you can, provided you back it up with good results!
If you don't increase the prices, customers can become disillusioned with how much it costs to get their windows cleaned and so when ones do hike their prices after many years, they nearly have an heart attack lol.
(I learnt that a while ago)