Thanks to everyone who replied. I’m happy to say its sorted and I have a current cert.
The flat’s consumer unit has always been protected with an RCD it was just the cable from the ground floor meter cupboard running to my second floor flat that was not. The first electrician failed this as it was not possible to determine whether the cable was less than 50mm depth in any wall. The second electrician passed this and said it was classed as a ‘limitation’. He also said 9 out of 10 electricians would of passed it but it is somewhat down to the individual doing the testing.