I have always used Shimano XT, so when it came to fitting the Stylo cranks last night I tightened the lock nut on the non drive side and this pinched the cranks against the bottom bracket resulting in them being very stiff to turn. I released the nut which allowed the cranks to move freely but it seems the bolt is loose and I am concerned that it will work free when riding.
Is this how they are meant to be fitted? Help very much appreciated.

i had problems with mine loosening off so i overtightend them and consequentially managed to snap the bolt at inners. don't want to scare you or anything but just watch out! prob use a torque wrench if you have access to one and find what they should officially be tightened up to.

if they're getting that tight and you don't feel like you're leaning on the allen key too heavy i'd be inclined to think you have too many spacers in there.
don't want to upset you intentionally but TBH i'd save yourself time and energy now, stick the damn things in the bin, and go buy another set of shimanos.
sorry.