Glennis,
There used to be a really good lab on the Dukes Park industrial estate, but I don't know if they are still going.
Chelmsford Colour Laboratories
18 Grafton Place, Chelmsford, Essex CM2 6TG
p: 01245451425 f: 01245451245
For this kind of work you really do need to talk to a lab first and find out what they can/can't do and see what recommendations they have.
You can get some good photoshop actions that simulate cross processing that may give you a clue what sort of effect you're going to end up with.
Here's the one I've used in the past
http://www.thelightsrightstudio.com/...Processing.htm
never done it for real in a darkroom.