... G4S engineer fitted the meter ...
Welcome to the forum artist.
The person who came and did the work from G4S was not an Engineer , trust me, he was almost certainly less qualified and competent than your electrician!
The other guys have made some good suggestions, I would concurr with them.
Get your Electrician to test the system again, dead, then he can apply temporary power, that will then prove that the heaters are working.
If they still don't work on E7, then you need to send his bill to the supplier and get them to send someone competent out to sort it out.
Stopping paying your bill might focus their minds, as might threatening them with OFGEM.
Though I think you have, unfortunately to exhaust all of the suppliers complaints procedure first.
Remember, though that whilst G4S may have been to your house, you need to get your electricity metering company to sort this out, as your contract is with them, and it is them who selected to use G4S, so if G4S can't do the work it's the fault of G4S, yes, but, the responsibility lies with your metering company.
Al a bunch of wazzoks the lot of em!