1 To pay or not pay?
If the service charge was compulsory and the restaurant won't accept your complaint, you can do either of the following:
Deduct some or all of the tip This depends on how bad the service had been.
The danger with this approach is that the restaurant could argue that you are simply trying to get out of paying. So leave your name and contact details with the restaurant and show proof of identity to try to make it clear that this isn’t the case.
It will be up to the restaurant to pursue you for the amount it says you owe.
Pay the full bill but say that you are paying the service element 'under protest' Write this on the bill and leave your name and contact details with the restaurant. By paying under protest you reserve your right to claim compensation later.
2 Gather evidence
Think about any evidence you could get, for example if other people are complaining about the quality of service see if you can get their contact details with a view to them supporting your claim. Taking photos of any spillage is another idea.
3 Write to the manager
If you paid under protest, write to the restaurant manager as soon as possible after your visit asking for a refund of the money that you are disputing. Set a reasonable time frame for a response (say, two weeks).
4 Last resort - small claims court
If you are not happy with the restaurant's response you could consider taking action against the restaurant in the small claims court.
There is a Pre-Action Protocol which sets out the steps you should go through before starting court action.
You should consider the value of the service element and disruption caused before taking your complaint to a small claims court. If the judge thinks the case is frivolous they could actually make a cost order against you if you lost.