Here is an article that describes the most recent legislation regarding tipping and service charges. It seems restaurants will now be obliged to pass on all tips,which will also be taxed.
I would think if you have had a particularly good waiter/ess, then tip them directly,and hopefully it will not then be dispersed amongst a dozen or more staff. This is important if the service was good,yet the food may not have been....they are two separate things.
http://news.bbc.co.uk.../business/8281191.stm