Thanks for the answers.
So, we have at least established that it is required to bond it or it may fail an EICR.
The pipe is definately cast iron and definately goes into the ground and has never been altered. A plumber told me the joins were made with lead. The pipe jutting into the bathroom has a lot of rust underneath that has been treated with rust converter.
What is the procedure to test it?
There are no flanges or similar places to attach to. I think removing a patch of paint down to bare, clean metal and then fitting the copper strip before painting over would be the best method to bond it.
Or, would it be possible to solder on a small metal tag with a soldreing iron?
What about removing a section of pipe outside and replacing it with plastic? (I may need to do this anyhow to fit a branch for a second toilet). Then it won't be able to introduce an earth potential will it?