Earthing and Bonding
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In a discussion on the uk.d-i-y newsgroup John Rumm gave this explanation of Earthing and Bonding, which is the basis for this article.
There is no such thing as "earth bonding": there is earthing, and there is equipotential bonding (main and supplementary), but they serve different purposes.
Earthing
Good earthing will ensure that in the event of a fault the supply can be disconnected quickly by providing a path for a large fault current to flow, hence causing rapid operation of a Circuit Protective Device. It will also limit the voltage rise of anything earthed that one can touch to a safe level (under 50V typically) during the fault. So if a live wire falls off the inside a washing machine and makes contact with the case, the big fault current will open the fuse in the plug or circuit breaker on the circuit quickly. Should a person happen to be touching the machine at the time, the voltage they are exposed to should be low enough to not place them at significant shock risk.
Bonding
Two types of bonding are recognised: Main and Supplementary
Main Bonding
Main bonding is the electrical connection together of incoming service pipes (usually water and gas or oil) to the main electrical earth (at the incoming electricity cable and meter). This ensures that (metallic) gas and water installation pipework running through a building are at the electrical earth potential.
Supplementary bonding
Supplementary (equipotential) bonding is not designed to clear a fault condition, or limit the absolute touch voltage. What it is supposed to do is electrically tie together any conductive parts (pipe or other services etc) that could under fault conditions introduce a dangerous potential into the room. Thus anything and everything that one can touch will be at the same electrical potential[1] as everything else - even if it is elevated way above true ground. So for example, without bonding a fault could result in hot taps being at 230V (say, due to being connected to a faulty inline water heater) while the cold taps are earthed via the rising main: this would pose a serious shock hazard since touching both taps would expose one to a 230V potential difference, causing electrocution. However if the taps are bonded together then both taps might be at mains voltage (230V) under fault conditions, but touching both exposes one to zero volts of potential difference. Equipotential zones only work where all "extraneous-conductive" parts (such as the earth wires of any circuits feeding power into the room as well as metal plumbing) are securely connected together and there are no conductive items or surfaces connected to earth.
Note that there is no requirement to explicitly connect supplementary bonding to the main earth terminal in or adjacent to the consumer unit. In reality you will often have a connection by default via the circuit protective conductors of any circuits that are included in the equipotential bonding.
[1] in reality the bonding may fail to tie all elements together at exactly the same potential, however it must limit any potential difference to 50V or less.
Plastic Pipework Installations
Generally with plastic pipe installations, supplementary bonding of the pipes is not required (although it may be required between the earths of, say, lighting and power circuits if they are both accessible in the room, plus any other extraneous metallic parts (metal Central Heating pipes for example)).
However if a bathroom is plumbed in a mixture of plastics pipe joined to exposed copper or steel pipework, the metal pipework does have to be bonded if it runs between the bathroom and some other part of the building, since a fault outside the bathroom could result in the metal pipework inside becoming live.
Installations not in "Special Locations"
In rooms which are not "special locations" (i.e. places likely to put you at increased danger from electric shock, typically because you might be wet), there is also no requirement. So for example a cloakroom with WC and basin would not require it (although it is commonly seen in such circumstances).
Change to protection for Special Locations in 17th Edition
Under the 17th edition of the IEE Regs (BS7671:2008) it is permitted to have no (supplementary) equipotential bonding in a room containing a bath or shower providing that the main equipotential bonding is in place, and that all the circuits feeding the room have additional protection from a RCD with 30mA (or lower) trip threshold.