10% is around my average, but it depends on the type of business I am in.
For example, I leave a tip in places like Kava, as it is a stand alone business, but never think to tip in Caffe Nero for example, as they are an international chain. (Poor staff still on minimum wage though)
There are some (local) establishments which are renowned for NOT passing the tips on to staff, but keeping them for the owner instead. I never tip in those places. (The Groes Inn is an example)