Thanks for all your suggestions. I have several balls in the air at the moment and unable to sit in front of my computer.
It's a tn-s system.
I haven't checked the conductor resistance yet but I will a bit later when I've dropped some of the balls.
As far as I'm aware, the minimum CSA for bonding conductors is 6mm² for TN-S earthing systems.
TN-C-S systems require 10mm².
When you conduct the adiabatic equation, you either use measured values and the disconnection times for those measured values.
Or use those from the tables in appendix 3, not a mish mosh of the two.
As far as I can see, the current required for a 100A BS1361 fuse to operate within 5 secs is 630A, 889A would cause operation within about 1.5secs.
As you haven't supplied the information as to the rating or type of device, a BS1361 100A fuse is most likely the worst case scenario.
Either use the tabulated value of 630A along with the time 5secs, or use the measured value of 889A and the time of 1.5secs.
Using 889A and 1.5secs will result in an earth conductor CSA of 7.6mm².
Using 630A and 5secs will result in a CSA of 9.85mm².
Bonding conductors are required to be half of the CSA of that required for the earth conductor.
Either half of 7.6mm² = 3.8mm², or
half of 9.85mm² = 4.925mm².
In any event 6mm² is more than sufficient.