This post was started a few weeks ago now, but if it's still applicable, you are not required to take the supplementary equipotential bonding conductor within a bathroom back to the consumer unit (or main earth terminal if it's not within the consumer unit). Regulation 601-04-01 states 'Local supplementrary bonding complying with Regulation 547-03 shall be provided connecting together the terminal on the protective conductor of each circuit supplying Class 1 (earthed) and Class 2 (not earthed but double insulated) equipment in zones 1,2 and 3, and extraneous conductive parts (metal baths, pipes etc..) in these zones'. You are not required to use 6mm cable; the recomendation is to connect all these parts together without breaking this conductor (stripping off the insulation without cutting the copper and looping in and out of a terminal or clamp). If you've ever tried doing this with a 4mm, you'll appreciate how hard this would be with a 6mm. The idea of not cutting/breaking this conductor, is to avoid the loss of earth contunity to the other parts if one connection was to become loose or undone.
I'm not disagreeing with breezer because there's no harm in doing what breezer said, it's just not a requirement, it's difficult to use 6mm and can be major work to get the cable back to the intake. Dare I say it, but the reg's have undergone some revision in recent years, and those techniques described are a bit old school, but there's nothing wrong with them.
I've written slightly more detail in one of the other posts by ChrisR about bonding in the electric forum. It is absolutely essential that you provide main equipotential bonding, I cannot stress this enough.