It's really quite simple, in your original picture of the rose there are 4 neutrals all connected together in the left hand terminal block ( one is blue going to the lamp,the other 3 are black). All three black neutrals must be connected together.
What you are missing is reading or acting on the information/advice which which has been given in one form or another 5 times.
Namely that the 2 wires which you keep describing as 'Loop' are in fact 'Neutral'. They are not and never have been loop according to your pictures in post#4 and you confirmed in post#7.
EDIT: I started writing this an hour before posting, in the meantime i've been sorting a boiler problem and hadn't seen Terry's reply.