You are right it generally does come from insurers and the minimum distance can vary from insurance company to insurance company.  Some will ask for 5m separation of combustible wastes and some will ask for 10m.  They will generally accept a shorter distance if factory yards are not big enough to allow for this level of separation but if you are in a high arson risk area or have high arson risk waste then they may ask you to have lidded, lockable skips or even move the waste inside at the end of the working day.   This information is usually contained in the conditions (warranties) associated with your insurance policy so you need to read it really carefully, your insurance broker should also be able to help you.

Graham - that is exactly the advice that I would have given you when I was doing insurance surveys.  Uncovered and combustible at least 10m and if lidded and secured 5m. Your Broker will quote you the same information as is in your policy conditions.  If they don't then I would be a bit worried because they could lead you into breaching a condition of your insurance which could invalidate your fire cover.    If you can't get 10m separation then you need to ask your skip supplier for a lidded, lockable skip.