The result was Not the Messiah (He's a Very Naughty Boy), a comic oratorio by Eric Idle and John Du Prez. Eric Idle sang "baritone-ish" and John Du Prez served as guest conductor for the NSO, as well.
About all I can say is that my face muscles are sore from laughing so much and my hands hurt from applauding.
I really hope this will be recorded. And, if you have the chance to see this, do go.