I'm prepared to be flamed but let me explain my logic.
Now DC are grown up, when we go out to dinner we are usually 4 adults. We don't do it very often but like somewhere "nice" so when we do, two courses plus drinks probably cost £150-£200. We'll be in the restaurant say 2 hours and the waiter will typically have 5/6 other tables (?).
That's an awfully good hourly rate if everyone leaves 10%.
Would you leave £20 on a £200 bill for 4 people? If not what is a fair/reasonable amount to leave?