As someone from well outside the UK it is apparent that several things stand out.
I am surprised that the CAA allows you to maintain a helicopter to LAMS. I thought that this was intended for older fixed wing types which had no manufacturers maintenance schedule.
The SIN on these clusters has been around since about 1975 and an FAA AD was first issued about the same time.
I would doubt that any of this scenario would have occurred had the aircraft been maintained in accordance with the Instructions for Continuing Airworthiness as required by a Type Certificate which would have included compliance with SIN's. This may have also precluded the attempt at a weld repair as the procedure is not documented.
It is tragic that this "problem" which is well known in the 300 "community" was not controlled accordingly.
Just because something is legal does not necccessarily mean it is safe. As to a CAA surveyor determining airworthiness of an aircraft I am afraid that is not and will never be the case. The LAME determines airworthiness, a surveyor can only establish compliance by the LAME with current regulations. You will never see an Airworthiness authority actually release an aircraft to service.
I have always been amazed at the attitude that a "fresh" 100 or annual is thought by some to be a guarantee of safety or continuing airworthiness until the next scheduled inspection. Nothing could be further from the truth.