As soon as you have purchased a site it is your responsibility to make it safe. Bear in mind that even if someone is trespassing on the land and injures themselves then you may be liable. If it is a totally safe flat area then you are in the same position as a farmer with a field but if there is anything that might pose a danger to (say) children playing then you need to fence it off and post warning signs. This will probably require 2m high steel site fencing or similar so that children cannot climb over it. Similar gates with padlocks will be needed and warning signs should be posted.
It is probably useful to plan any such fencing with a view to it continuing in use throughout the building process when all the area with building works on it will need protecting. It may also be worth thinking about future security issues at the same time. This will involve checking the cover available from insurance companies for theft and third party liability.
There are some specialist insurers dealing with self build projects. Quite a good list on MoneySupermarket