Fixed price building contract - anyone done one?

[TuttiFrutti]

We are planning to build an extension, and friends have told us that it's better to go for a fixed price building contract so we don't get any nasty surprises sprung by the builders.

Has anyone done this? And how do we go about finding the right form of contract?

Nobody?

Problems with any fixed price work...
There's Celia's neighbour, who to an extent got off lightly. A lot of building work is stuff you can't see.
Also you often want changes to the original spec, and thus "fixed" often ain't fixed.

If the work is substantial, then it might be worth paying for someone to manage it for you.