I've done a few mods to mine each time producing to the CAA a copy of the FAA STC, the CAA then accepted the mod as a minor mod without much difficulty.
The FAA web site has a STC search section.Very useful for gleaning info about substitutes where an original part is no longer made.
The Americans also have a "field approval" and something called a Form 337 or 137?, I dont know how those equate with our system.
Your engineer should know what the CAA are likley to want.