Can I use screws / nails to affix items to the floor on top of the heating system?
No. Do not affix items to the floor using nails or screws. If the cable is damaged the earthed screening will protect you by tripping the RCD but this damage to the cable will mean the system does not work. It is important to plan your installation so that you do not put the heating system beneath areas where objects need fixing e.g. Toilets, Kitchen units etc.