The thing is there is no need to ask.
You have made the decision to increase the price. The customer will either accept the increase or decline your services.
Im with smudger on this one.
Not knocking you james, well done for putting your prices up.
I'm a coward now, done all letters, explaining on the door.
Most people are ok but does leave the door open to a debate or every other month

So i simply put on my bill new price £___ (i.e 50p on fronts £1 extra all round) and i wait til custies out and leave increase on bill in s.a.e.
Instead of having talk on door.
As said cowardley i know but if custie has ump over price increase usually forgot next time they see you
or deal with fall out then.
As said dealing face to face can feel like your asking permission or get into explaining why it's gone up.
Can't be doing with lenghty letters explaing why also; if you were sticking price up £3 plus then yes.
Lee ps. price increases around 18 months, i think every year can seem a bit much in my opinion.