I wouldn`t recommend doing the separate earth via the earth rod system if the earth supplied by the electricity company is a Protective Multiple Earthing (PME),

http://www.rsgb.org.uk/emc/emcwebsite/leaflets/emc7protectivemultipleearthingmembers.pdf

With a (PME) the Earth is at the same potential as the neutral, so in effect you could be putting a fault on the circuit and this could trip your (RCD) residual current device if you have one.

http://www.tlc-direct.co.uk/Book/5.6.1.htm

Meridian 800 DVD V4, DSP7200.2's