Ok, update....
The CPS is alright after testing, the earths have been cleaned and we are still having the same problem.
We are thinking the next step is to send the ECU to a company called ECU EXPRESS in Stoke, who will fix the relevant part of the ECU that is not sending a pulse to the injectors. They will fix it and return it back to me for the sum of £400.
We are thinking that all the the "fiddling" with the battery will have caused a spike that will have damaged that particular part of the ECU. If this is sounding completely wrong to anyone, or if anyone can think of anything I can do before shelling out £400 whilst on Furlough, I would be very grateful!