I am not a domestic electrician, I was always industrial so may be wrong, but this
Seems wrong for a new consumer unit, most RCD's have
or similar printed on them to show what type they are, the sign shown is for type A, not sure the date when this started, but my very old RCD's like yours don't have any type marked. Today we have type AC, A, F, B, S etc. Although type AC are no longer fitted, it is rather technical down to amount of DC and frequency and delays, which you don't need to know about, but fact the info is missing points to this being a rather old installation.
Sounds like a 'Borrowed' neutral.
I tend to agree, seems very likely the electrician has made an error. Since around 2005 electricians doing domestic work have needed to provide minor works or installation certificates which are submitted to either the LABC or Scheme provider to get the completion certificate or compliance certificate depending on the route taken.
If the electrician displays any of the scheme providers logos, or shows in any way he is a scheme member like internet web page or sign written van, then he is responsible to ensure the work is registered, unfortunately if he don't then down to the home owner to register the work, however it is clear a home owner can't do that without the installation certificate so non issue of the certificate does show he was doing some thing he knew was not legal.
There has been a case "Pembrokeshire trading standard EICR court case v Mark Cummins trading as M C Electrics from Haverfordwest" where an electrician was prosecuted for not doing the work he said he had done, however there is nothing to say what the home owner got from this. He got fined, but that does not go to home owner.
I had a problem with my mothers house where a builder ran off and would not correct faults, including electrical, and thinking it had all been done up to that point legal, I contacted the LABC to say I was taking over the work, it seems the builder had failed to register the work, it caused me even more problems as a result, and the priority was to get wet room up and running. But our attempts to get money back from builder failed as LABC got their first, and builder stopped trading.
So threat come first, yes trading standards may take him to court, and stop him doing it again, but that does not help you. What is more likely to help is contract his scheme provider, they may put a boot up his back side, however they are not likely to admit that to you.
But as said that RCD seems like old stock to me, my old house no logo fitted 1992 with second hand RCD, not sure when the logos started to be printed on the RCD, maybe others can help.