That raises a good point –.there is absolutely no need to tip extra if there is a service charge added to the bill. Check on the menu in restaurants, and also on the bill. The fact that the credit card chit might have the total blank and a space for a gratuity to be added doesn't necessarily mean that the service charge hasn't been added already.

Many hotel charges also include a service charge.

Now, if there is not a service charge, that doesn't mean that you have to tip, but it's definitely surplus to requirements to do so if there is a service charge.

ETA: I'd also add to travellers from the USA: don't worry too much about it. I have been with Americans at dinner in London who seem to spend half the meal discussing the tip and then continue to discuss it for half an hour afterwards. It's not a big thing in most of Europe, and there's no call to let worrying about it spoil your trip!