Does anyone know if there are any rules/regulations or best practice guidance on how high empty pallets are to be stacked (outside).

Down to Risk Assessment given pallets can vary in footprint, condition and load capacity

AND

any specific insurer instruction regarding stack height for material especially combustible wood or fibreboard with respect to proximity to buildings and perimeter boundaries.

You could try the search function

http://forum.iosh.co.uk/posts/t133001-Outside-pallet-height-storage

http://forum.iosh.co.uk/posts/t106494-Pallet-storage---advice-please

http://forum.iosh.co.uk/posts/t87116-Empty-Pallet-Storage-Height

In my experience, your first call should be to your insurer (as per Roundtuit).  They will have very clear expectations of either how few pallets can be stored in the location you want to use or how far the area needs to be from buildings for you to store then number of pallets you want (dont be surprised if this distance is significantly outside your site boundary).

Wooden pallet stacks are the perfect fire potential.  Loads of fuel with loads of air trapped in it and clear heat and air pathways.
It will sound a bit defeatist but of the 3 or 4 of these conversations I've been involved in over the years there was always a massive difference between the site and insurer expectations.