How confident are you?
How professional looking are you and your set up?
Do you have a quality web page to refer to?
I ask these, as you said you havent ventured into the higher end of the game yet with these types of property. You have to look the part and then prove that you can deliver. So your price needs to reflect these things. As someones earlier post mentioned, people who live in those properties are used to paying more, ie ultility bills, council tax etc etc. They also pay for quality and expect both and also have an image to upkeep so would want professional looking people at their property.
Sell yourself and the high quality and advantages of wfp to them. The price then reflects this. I would go in no less than 90.
How many of those semi at £15 a pot do you clean in an hr? If they are in the same street as most of mine are, im guessing within around 12/15mins finished one and starting next. So if averaging 60 quid in mediocre areas within the hr can you not understand how in a much nicer area and different style windows that you need to be charging way higher than £60.
(A major silly comparison, a 1ltr bottle of water from your supermarket cost how much? Then price it up from say a local chippy, then a restaurant and finally a posh restaurant! Same water, but slightly different factors and the price can double, tripple or more!!)