I would use 10% as a minimum and then gague how much more depending on the service I received. I would not tip if I wasn't happy with the service, obviously. I would also like to mention, don't forget to give the assistant a little something, even just 50p as they really don't earn a lot and it is very hard work.
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dear Isabel, the tip i's always up to you, what i think is that when you tip a lot the stylist will remember next time