May be of use with regards to earthing systems and values with regards to TT systems.
http://www.eon-uk.com/distribution/CiCdocs/01%20Technical%20Documents/CN%20East/East%20Earthing%20Manual%20Feb%202002.pdf
http://www.eon-uk.com/distribution/CiCdocs/01%20Technical%20Documents/CN%20East/East%20Earthing%20Manual%20Feb%202002.pdf