I would say that raising prices by 50p anyway is a bit of a joke! Why bother with small incremental raises only to upset them next year. I put my prices up every 2-3 years or so, but when they go up it is by £'s not p's. I know shiners who puts them up 20-40p each year. They started off with good paying work but after 10 years or so they are behind the decent rate, "that will be £11.40 madam" odd really. Especially when the price should be £15.
If your going to "upset" them you may as well get it over with. Remember if prices go up 40% and you loose 40% you are likely to earn the same in a shorter time.
Plus dont try the inflation comment, it has never been lower. Demand of your time is the reason for a price hike. if they really want you they will pay. If not they can have "one scrim Bob" (That is to presume you do a top quality job and guarentee your work. The £11.40 guy does a shoddy job and has the manners of a mule)