I've got a XC mtb which 1. I'm trying to cut the weight off , 2. I want a bit extra reach. I've got a longer road bike stem made by 'Deda' (I think Planet X)? and I was wondering if it will be strong enough for XC mountain biking use?
For XC and trail use it'll be fine.
It's really only downhill bikes that require especially strong stems, I think (and in that case you'd likely be using a shorter stem anyway for handling reasons).
It depends on your bar diameter. Oversize bar is 31.8mm for both road and mountain bars, so the stems are interchangeable.
For historical reasons, most of "standard" drop bars and stems for those are 26.0mm and other bars are 25.4mm. It is possible to tighten 26.0mm stem to hold 25.4mm bar, but it will put extra tension on both stem and bar. Mountain bar can be used on road stem with a shim that can be made of soda can, but mountain stem will crush a road bar.
 The international standard is 25.4mm and 26mm is Italian national standard, but since Italy used to be major exporter of high end road bikes, that is the more common one. Deda is an Italian brand, by the way.