It has been a long time since I did any testing on them, usually easier to get those who install them to do the testing. There is a very large portion of the testing which is visual, all connections have to be visually checked for any corrosion, the outer covering on the conductors themselves have to be inspected, and the clips that hold them in place, there should also be a label, normally at around normal head height indicating its purpose, some have the installation date on them. If it is safe to do so, they are disconnected from the MET, and tested as you would an earth electrode, along with continuity testing, the results of these tests can then be used to calculate the amount of voltage the conductors can carry safely to earth. It has been years since I did any, but I think the voltages involved are so large that it makes your eyes open wide when they tell you.
The regulations will have changed since I did anything with them, BS 6551 was replaced in 2006 I think, with BS EN 62305, not too sure, I only know that because of the CDM requirements of a job I did recently made mention to them. I remember a firm doing some testing for us at one point, they were from the Midlands not sure of the name (quick google) Test strike UK they were called. Afraid thats all I can the advise I can give.
