Mike-- consult a good "how to" book that reflects the practices in the UK. Improper grounding of the neutral condutor can cause many safety issues for the folks in your home. Inadequate grounding can even cause stray current to appear in your neighbors home. Dont do anything unless you understand exactly how to do this job!
If you attempt to do this work your self be aware that current may be circulating in the grounding system and may cause a current to flow across your body if it is placed in series between two "grounded" obljects.
I dont know what your local rates are but I wouldnt think that a good electrical contractor would have over 3 hours tied up in doing a job like yours. It may be worth the cost - remember he is bonded or insured and responsible for any damage an improper job may cause. Not installing a ground (earthing) system correctly can cause a multitude of hazards in your builiding.
HB ( a partial list of my credentials:over 20 years exp. as - Electrical lineman, Troubleshooter and licenced Electrician.)