As you say, it sounds as if it would probably be hard to justify, BUT ....I am just looking through the cost breakdown page .... for labour it has 23.50 single time hours ( half hour doing the job and 23 hrs sitting in van drinking brews lol ) ... 3 meter of 35mm2 cable (which is incorrect no cable was used) ... how can you justify the above for terminating a open end cable and burying it in the same location
... if I were you, I would steer well away from even mentioning, let alone arguing about, the amount they are trying to charge, since they would then probably get the impression that you would probably agree to pay something. I would have thought that your argument should be that you should not have been billed for anything at all, whether the job 'cost them' £5 or £5,000 to do.
... just my opinion, of course!
Kind Regards, John