Ian,
Here's a link to the Air Accident Investigation Branch at Farnborough. It's an arm of the Department for Transport.
Air Accidents Investigation: Home
In the first instance I'd write to Keith Conradi who is the Chief Inspector.
There is also the National Archive (formerly the Public Records Office) at Kew, West London. You have to employ a "reader" to gain access to any particular file though I don't know the cost or how to go about it.
SGC