This article is from the Real Ale FAQ, by with numerous contributions by Brett Laniosh others.
A: Yes. If you ask for a pint you should get a pint!!!!!!!!! At the moment
(although the law may change on this soon), a test case has shown that
pubs can serve a pint up to 5% short, but if you feel that you are being
short changed then INSIST on a top-up.