Where has 10mm2 come from for gas pipe bonding. If my system has 6mm2 and was fitted ages ago by YEB what /who says it must change?? and why??

Your bonding must be prior to 1977 as its been 10mm since that time as published in the IEE wiring Regulations which is now BS7671 2008. I would recommend you get a full Electrical Installation Condition Report carried out asap, as your installation sounds like its old and no longer compliant with current regulations and stanards. I would also recommend reading this free leaflet from the Electricity Safety Coumcil; http://www.esc.org.uk/fileadmin/user_upload/documents/public/leaflets/E_B2011webA4screen.pdf

Its regs my friend,Abide or dont moan when you fry to death in the middle of the night.

the reason why is that in the avent of a short circuit of a live cable to the gas pipe, say a trapped cable under a floor board, for a split second we now know upto 6000 amps can pass through the gas pipe to earth, so we want the breaker to trip as quickly as possible as the pipe is full of gas! so we make the path of current flow have the least resistance possible ie the cable bigger so that if it happens it only the breaker and not the house htat blows