H
Heisenberg
Part P does not cover inspecting and testing.
Part P only covers electrical installation work.
Napit only insist that you have public liability, I assume because you are a member of their Part P scheme, which by virtue of being Part P does not cover Inspections and Testing of existing installations.
However I feel fairly certain that if you were to join their scheme for Insprction and Testing, they would insist you have professional indemnity.
The OP is with Elecsa, not Napit. However, you are correct, Napit want you to have 250k worth of indemnity and have your 2391 or equiv, if you want to test and inspect.