ACX thanks for the information.
BUT this doesn't answer my question - what would I have to do to become approved to change the webbing on a seatbelt? Surely, it wouldn't be necessary to be a part 145 organisation, but if it is then what kind of process would it require?. I know that approved welders just have to submit test pieces to a test-house but as far as I know they don't have to be a part 145 organisation to do welding work.
I'll try and pose my question to the CAA - see what they say.