ive noticed by non-drive side crank had some play in it, i loosened the clamping bolts and tried to tighten it via the plastic nut and its will barely tighten without popping out the thread…..

– Is it knackered?
– Can i buy a replacement plastic but only?

How much are you trying to tighten it? There is a torque on there which is VERY low for a reason. It is only a bearing pre-load.

And yes you can buy them for about £2-3.

http://www.ukbikestore.co.uk/product/118/1f8_1100/shimano-m760–m800-crank-arm-bolt.html
http://www.chainreactioncycles.com/Models.aspx?ModelID=5095&utm_source=Google&utm_medium=Shopping&utm_name=UnitedKingdom

Is it broken? Very easy to overtighten and break those plastic bolts. You can buy metal ones, but they make it even easier to put too much preload in the bearings!

is the plastic bit only just connecting with the threads. Or is it tightening all the way and poping off.
Either way you might have a issue with spacing. if your chain line is good leave the drive side and adjust the non drive side.