Power tripping when shower on…

if a replacement shower was recently fitted then you should have gotten a certificate for this. It’s unlikely that it was tested as it wouldn’t pass the certification process with the setup you described. Most new showers also use more power and the cables feeding the shower should be upgraded to 10mm to facilitate this. Someone linked a previous thread describing a serious safety issue because the new shower wasn’t done correctly.
post a picture of your fuseboard.

Does the tripping coincide with prolonged shower usage, I have come across a few instances of where houses with single RCD protction was tripping the RCD but not the dedicated shower RCBO after say > 20 minute showering due to the higher level of moisture ("steam") presumably getting into the bathroom light fitting.

you didn’t get a certificate as you say a plumber connected it up(reputable company)

that’s the problem

No plumber would do the tests that a rec should do (or have the equipment).

A rec would have spotted that the shower circuit wasn’t Independent

Also a rec would check if existing cable was capable of taking the extra loading of your new shower

along with earth testing and rcd trip times.

Old shower was most likely 8.5kw on 32amp trip switch. Connect a 9kw shower to the same fuse and it can cause tripping.

The new shower could cause tripping of the 32A MCB after extended use of the shower, this shouldn’t be a factor for a tripping RCD though.

It used to be permissible to protect the shower with the same RCD as the sockets, and if it complied at the time I don’t see any reason why it this in and of itself would necessarily need to be changed. A separate RCBO doesn’t add any additional safety mechanism.

Although given the relatively modern vintage of that board, separate a RCBO likely was a requirement at the time, and it was combined with the sockets RCD because the board was full and single mod RCBO’a weren’t as readily available then. Given that the cable is probably a little undersized, now might be a good opportunity to sort all.

Boderline low impedance between neutral and earth on any of the RCD protected circuits would do it. When the shower is on, some of the relatively high neutral current will take a parallel path to earth downstream of the RCD and cause an imbalance.