From what I can gather, if the disconnection times are met, all circuits in the location are RCD protected and the resistance is less than 1666 ohms between extraneous conductive part and the MET then it is considered effectively bonded.
Still not 100% sure on the last bit as I am thinking something outside the location can still cause potential to be introduced into the location via the extraneous conductive part.
If the resistance is 23k ohms or more then it isn't considered extraneous so doesn't require bonding anyway.
You're getting there.
First off, the 2
2K-ohms comes from GN5. A part exceeding this resistance, measured between the part and the MET is considered to NOT be an extraneous conductive part and therefore does not need supplementary bonding at all.
In practice you're likely to get either very much greater or very much less than this value.
There is no point in creating extraneous conductive parts by unnecesary bonding, so, if all circuits are 30mA RCD protected there is no need for any supplementary bonding at all. (The RCD will ensure that disconnection times are met and parts are either extraneous, but already bonded via MPBCs or not extraneous and need not be bonded.)
The 1666 Ohms is where you're getting mislead:
If not all three conditions can be met (e.g. not all circuits are RCD protected and you are modifying only one circuit) additional protection can be provided by supplementary bonding. Supp bonding can be carried out by a combination of CPCs, conductive parts or additional supp bonding conductors.
To decide whether you need to actually add extra green-an-yellers you measure the resistance (R) between accessible conductive parts in the location. If the measured value is less than 50V divided by Ia, then no additional bonding is necessary.
Ia is the largest value of the current causing automatic operation of the CPD of any of the circuits in the location.
For instance, if you have a 6A Type B CB on the lights and you have a 40A shower circuit, which is RCD protected, then Ia for the lighting circuit would be used and your R would have to be less than 50V/30A = 1.67 Ohms to avoid bonding.
It would only be in the case of all circuits being RCD protected that the value of R comes out at 1667 Ohms... in which case, you would not need to bond anyway, for te reasons already explained.
The only thing to worry about would be if you have extraneous conductive parts not bonded to the MET, such as metallic waste pipes going into a cast iron stack... but this should have been subjected to main bonding.
Anyway, after all that I have to say that it's all bolleaux - the only way you're actually in danger in a bathroom is if you're carrying out naked, wet, live electrical maintenance, in direct contavention of the requirements of the Electricity at Work Regulations 1989...
...and I tell you, fellas - never again!