Hiya
There are two implied questions here... will Defender tyres fit your alloys and would you have any problems...
I have 235/85/16's on standard alloys and they fit fine (and, indeed, are a standard Defender fitment anyway...)
However... if your Disco is standard (as opposed to teh rims...) you may get some rubbing - as pointed out there is plenty of information on this including a sticky at the top of the page! Mine has a 2" lift and I only get rubbing on the wheelarches at maximum articulation although you will need to wind out the stops to prevent the oversized tyres snagging on your radius arms..
Hope this helps