What is the cheapest way to see a London show?

[Lilliput]

Our summer holiday this year has been restricted to 3 days in London in July. I thought it would be great to see The Lion King or Oliver. I have trawled the internet for tickets but they still seem pretty steep.
How do you get really cheap ticket?

Lastminute.com have a few offers?

Ticket booth on the day for returns, at theatre or Leicester Square, chancey though. You can check here and sign up for offers by email newsletter but offers for the really high demand shows are rare. Otherwise there is Kids Week but that is too late for your trip(late August).