Five months ago I took over a row of four terraced (large terraced houses mind) and charged them £12 each. Their previous window cleaner who dropped them was only charging them £6.00 each (didn't you Squeaks

).
He reckons I should be on Rogue Traders.

I do a lot of work in this same area (about 8 days per month), and it's one that's avoided by many window cleaners; access is difficult; nigh impossible with a van-mounted system; and it's difficult to do with ladders too.
It's ideal territory for a backpack and a trad partner, so I charge well in this estate; though I do have some underpriced customers from when I first started, but they're nice customers, so I don't mind too much.
Round here I think £8 to £10 would be fair for an 'average semi', though I do know guys who charge a lot less, and others who charge a lot more. My cheapest semi is £6.50 (I haven't put the price up in five years, and my most expensive is £25; 'cos it's out in the sticks (it has a small conservatory too).