That form has space for the import agent's details and stamps etc.........what you need is their completed copy of that form, not your orginal, half filled out one. I can't believe that it wasn't given to you at the time.
When an item is imported from outside the EU, unless it is an aircraft flown in under its own power, the import process is very formal. You won't get your stuff off the docks unless, and until, the process has been completed and an "import" has been accomplished. The resulting documents from that process are what you need........I suggest you contact the shipping agent to see what they can dig up for you.
The 'certificate of free circulation' was dreamt up by some more than usually helpful public servants to save their taxpayers time and effort 'proving' status every time some foreign uniform junkie kicked off. The dogs breakfast that is EU VAT law means that there are many situations where free circulation is difficult to prove (or disprove) and the law now provides no help for the law abiding to 'prove' their position.
Most aircraft owners do not help their position by establishing a paper trail that justifies their free circulation status and plenty of useful paperwork has been binned or lost because nobody thought it was worth keeping.........