Just to try sum things up for ya...
If its main Bonding that should now be done with 10mm to within 600mm of the incoming water supply to the house. Some older instalations were fitted with smaller bonding cable tho...
If its supplimentry Earth bonding your doing then the aim of this is to make sue you have a continuose link back to earth from any electrical item in your bathroom area.
The (Main bonding) conects the main pipework to earth, thus all you need to do is add (supplimentary bonding) to the current pipework in your bathroom (as long as theres no plastic joins / pushfits in the pipework your ok)
Best to make sure you have both bath pipes and if bath is metal bonded together, same with sink, and radiator. then you also need to bond the electrical earths to this curcuit.... Ie your light switch earth, shower switch earth and any other item thats electrical that is placed in the bathroom, or adjoining boiler.
Dont worry if you have an electric shower, as long as you bond the Showers switch, that will be fine, as its part of the same cucuit.
REMEBER ITS EARTH TERMINALS YOU ARE BONDING TO THE PIPE WORK....and NOT the + or - terminals.
Well Hope thats helps, took me some time to get my head around it, but think i got there in the end with the help of JohnD and others on here.... Also helps to buy the odd book on electrics too... Its amazing what you find out with a little investigation.
Feel free to shoot me down guys if im off the mark with any of these comments....