>I am told that a group of twenty or so (I think) people will contain at least one example of a common birthday (day-month only, of course).

You're on the right track, lusy, but the word "will" is dangerous, statistically speaking. Ignoring 29 Feb birthday deviants, you'd need 366 people to be certain that two would share a birthday.

A group of 23 or more people has a better than even chance that two people will share a birthday. Maths teachers love testing this out on an unsuspecting class, because with a class size of 30, the probability of a shared birthday jumps to 70%, with 40 pupils it's 90%.

I won't go into the maths behind it, but you could check out http://www.maa.org/mathland/mathtrek_11_23_98.html