Sorry if this is a stupid question, but does anyone know how to measure a MTB stem please?
i.e. where do I measure to and from?
eye to eye, the entire length or some other way?

I think it's the middle of the top of the headset to the middle of the bars.
Ah just found this, which seems to back it up
http://www.cyclingnews.com/tech/fix/?id=position

Mechanic Adam says

From Allen key bolt on stem cap on top of headset, to centre of clamp on handlebars.

so are you building up that new bike then?

Fab - cheers guys!
Yes looking for parts for my new frame. Going to America next week (business not pleasure unfortunately) so doing my research now for all those bargain parts I'm going to buy ($2 to the £1... can't wait!)