I've just come from the Londoner - after all the recommendations I read
on here, and I have to say: wouldn't recommend it myself.
I had my roots touched up and a trim and it cost me 208 CHF.

A little pricey - but ridiculous when you consider: I had to re-wash my hair when I came home because although the hairwasher ran water over my head for 10 minutes, she didn't actually rub any of the dye off. I had to remind her that she hadn't even done the back of my neck as she was trying to put conditioner on.
And the stylist trimmed my hair whilst it was dry and styled (??)

I see now, that I've gone back into the forum that most of the reccos were some time back.
You won't die or come out with green hair, but I suggest looking elsewhere.
C-