build to boundary guttering overhang's neighbours property

Could anyone please tell me who is legally responsible when a newly constructed house has been built with no setback allowance for guttering on a build-to-boundary garage wall, as required by building regulations.
As a consequence, the guttering is overhanging the neighbour's property.

Draughting plans show a 150mm setback allowance was required.
Structural engineers were engaged
The site was laid out by suryeyers.
A private building certifier was used.

The property was inspected and passed at:
Foundation stage, excavation stage, slab stage, frame stage and final stage.
A final Inspection Certificate was issued.
The builder says everything was done using professional people, so he can only go off what they have provided him and that a Final Certificate of Occupancy has been given by the Certifier.
I'm not sure where I should go from here.

Basically you contract is with the builder so its his responsibility to provide a remedy.

If its the surveyor who caused the problem its up to the builder to claim the cost of the remedy from the surveyor.

I would be contacting a solicitor for legal advice if I were you. Good luck.

Modifying the wall and truss to accept an internal gutter (eaves gutter inboard level with the brickwork) should be possible and not too expensive. Your builder should pay for it. Unless that is the plans and drawings actually showed the wall on the boundry and an external eaves gutter in which it would be your responsibility or at least in part.

builder should be fixing that by changing overhang gutters to boxed gutters on top of boundry garage wall

I wouldn't get distracted by listening to the burblings and excuses of the builder. Your contract is with the builder, the product he's delivered is obviously and indisputably incorrect and so he needs to fix it. Don't be the one looking for the "culprit" in some chain of trades - no one is going to put their hand up.

I hope my neighbour cares as much as you do - they're garage gutter is sitting right on top of ours and we paid for ours to be done correctly with "s" flashing...

^^^

so when it over flows it drains onto your gutter? Totally illegal.