Before investing in plastic fencing/chicken wire, I suggest you check your allotment tenancy agreement. There may be a clause which prevents you doing this. However, having said that, are other allotments marked out in the same fashion? Speak to your neighbouring allotment holders. The long steel spikes you refer to are called 'road pins' see if you can contact a civil engineering company as road pins no longer comply with NRSWA (new roads and street works act) and they may have some spare