I'll kick it off... how can he make the statement "Inconsistent resistance values for ring final circuits" when he doesn't appear to have actually tested/recorded them?
Given it's a studio flat, are there too many points of utilisation for him to count and record?
It's unlikely you'll get identical results for the various insulation resistance tests as he has recorded, unless you max your meter out (so I very often write >999 in all IR test results).
Max disconnection time for the shower circuit... it's a final circuit so it's max disconnect time should be 0.4s, not 5.
Can't see what's wrong with the fusing of the under floor heating... the breaker is 16A and it's wired in 2.5mm... nothing wrong with that. And at the other end I wouldn't expect anything else, just a connection to the under floor heating controller.
The rest of it, difficult to say without seeing the installation, but I stand by what I've said already... he's a chancer, trying to extract as much money from you as possible and I suspect he's in cahoots with the agent. Ditch them both.