[The Saucepan Man]

Esty, I have always found that the easiest way to book tickets for London shows is through one of the on-line booking agencies (for example, See, which is linked to in the link that davem provided). You have to pay a small surcharge, but its usually straightforward, painless and quick. You can also often click on the map of the theatre to see where the seats on offer are.

It's worth shopping around, though (I usually simply Google something like "London tickets"). Prices can vary (due to the variable booking fee) and, if one agency doesn't have tickets on the day you want, others still may.