12.5% for parties of 4+ (from Christmas booking terms)
12.5%Young's own Christmas booking terms state a discretionary 12.5% service charge is added for parties of four or more, and for every party on Christmas Day. These are the only terms on its own site that mention a charge, so the rule outside the festive season isn't published.
Source: Young's's own published policy. Service charges in the UK are discretionary: you can always ask for one to be removed.