TT is where the ground is used to transmit the earth, with a generator there is no need to do that, but the problem is with any emergency power supply to ensure nothing dangerous is sent back down the supply line when the back up is in use, so when the DNO worker has isolated and earthed the supply at the origin nothing if fed back from the destination which can injure the worker, this includes using the DNO earthing system once the supply has failed.
The advantage of TT is there is no worry over the DNO worker, but the disadvantage is the RCD is then the primary safety device. As a secondary device no one really worries too much if type AC, A, F, or B, but as primary device that changes.
We also need to consider a gradient between earthing systems, being able to touch two earthing systems with over 50 volt between the two must be avoided, and the design must take this into account. This has caused a big problem with electric car charging, and there have been so new methods used to get around the problem, which 20 years ago would not have been allowed.
I have at work many times been approached and asked how do you do this, and I have given an answer, then maybe 4 months latter visited the home, and realised the description had been warped some what, I the result was not what I would have considered safe.
So you have had a guy on site who has installed an unsatisfactory system which can fail causing flooding and he said this is required to comply with regulations, this does not seem correct, there must be a method where the pump can do its job, we see with life support equipment how in some cases the fuse is not fitted in the 13 amp plug, where some one has done a risk assessment and said having a fuse is too high of a risk.
So it seems likely some thing can be done, and with remote pumping stations we have telemetry to report on faults and special RCD's which can auto reset, but the big question is, on a DIY forum can anyone safely advise what to do? I would say best advise is to get some one on site use to working with this sort of equipment, when I worked for north west water, and SLD pumps we had the infrastructure where we could get specialist advice, including the supply of specialist equipment. So your looking for a firm with water or pump in their name, who can send some one out to advise.
To use advise from the forum which includes me, could result in danger, either from flooding or from electric shock, it is just too easy to imagine what you have and get it wrong.