I've been Window Cleaning since 1991 and have been given a few underpriced rounds in my time. Ones where the previous cleaner has done them for too long for next to nothing. I have tried all methods to get up to the right price. By far the best way is to be honest, tell them what the price should be and why the previous cleaner prob wanted rid. Then give them the actual price it should be. If that is £12 so be it.
Whatever you do don't think that you can put them up £2 per year for the next 3 years. Both you and the customer will be frustrated, you because for 5 years you are not being paid properly. Them because every year you put the price up. The problem with this is that you will be putting them up 3 times by a high %. They will ask why, if you are happy to work the first year for £2 more, you need to keep increacing by £2.
I have had less problems doubling the price, explaining that they were not correctly priced, than trying to creep the price up over time.
Plus if you get a new customer, do you keep it at the same as her neighbour or do you give her the correct price. As neighbours talk, if the prices are different, it takes some justifying.
In conclusion, be honest with the customer about how much it should be and charge accordingly.