aftermarket / scalper ticket purchases in London?

We're currently contemplating a family trip to the U.K. Among other things, my kid would enjoy seeing the Harry Potter play. I expect to pay a mint, but I'm curious what the preferred solution is for buying tickets when they're already sold out. Stubhub seems to be a big player in London. Are there others I should consider?

Stubhub's the only one we've ever used, they were OK