Does anyone know of a site detailing engine / gearbox compatability ? If not, will a gearbox / bellhousing from a sierra fit a 2.0 litre Zetec ?

Yes I do and yes it will. There's a link from chrisg's site or go to the Burton Power site or better still there's a full page in their latest catalogue with all the engine/box combinations.
http://www.gibbs111.fsnet.co.uk/eng_gear_comb.htm

Hey Mix, welcome to the site.

If you wants lots of info on the Zetec, check out www.zetecinside.com. Lots of useful stuff including gearbox info (and I think you will find the answer to your question is yes)

I phoned burton up on this matter a couple of weeks ago, They said, the original bellhousing on the type 9 gearbox will fit straight onto the zetec block.

Only changes would have to be to the flywheel and clutch. I think around £100 for the lot including release bearing.

Price could be wrong though! Best check!