Hi. We sold our former UK home October last. I considered removals companies, but in the end for various reasons decided to organise it myself. As a result I have made two trips so far via Calais with vehicle and trailer full, emptying the lock up I then rented. When packing everything up we decided the easiest way to record contents (which were a mix in some boxes) was to photo each layer, using a card index card with a unique number.
I wanted the reassurance of having the inventory covering all the loads accepted by the Douanes. I prepared a spreadsheet for the Douanes with the following headings:
the numbers for the boxes of paperbacks are those written on the boxes and on the index card inside. I have the photos on a memory stick etc, just in case there is a challenge and I need to show what is in the load (ideally without unpacking).
(I used a few more headings, but did not print them for the Douanes… these included my estimate of insurance/replacement value [more than the value I put for the Douanes] and dimensions). I made sure that there was a header and footer on each page of the inventory with date, page number, signature, place signed etc, as well as a heading “inventaire de XXX {address of property sold}.”
I read fairly extensively and did find references to the need for the costs in the inventory to be realistic (or the Douanes will estimate the value themselves) but I took these in the end to be the price I would get in a fire sale, not the replacement values or insurance values!
I came Dover - Calais.
Customs at Dover also regularly check vehicles, particularly if they look laden, so there is a delay if you are signalled to go via their shed. They appear a bit reluctant to open things if there is a risk that everything will fall out! Confirming that risk seemed to speed the process 
The Ferry operators confirmed to me that its the responsibility of the Customs and of the Security staff at Dover to alert French Customs. So trying to drive through Calais port without reporting in is probably an unnecessary risk, although it seems quite feasible.
The customs post in Calais port is not simple to find, but is signposted (mainly for commercial trucks), and the lane is currently marked in yellow. The area is cluttered with parked lorries. There are some sheds for unloading lorries backed up to them, with roller shutter doors, then an office up stairs and accessed by an open balcony. With Covid restrictions, they limit the number of people at a time, so there are often drivers hanging around the steps, waiting, (nicer if its warm and not wet!). The douaniers sit inside behind a glass screen, in booths. Once inside you wait for the next one to become available and notice you! They do use a mini translation device, but it helps to explain to them in French what you are doing there and why. They are not accustomed to this process, and spend some time looking it up online. That in my view actually helps, because they are so absorbed in the mechanism they do not query much provided the paperwork is correct.
The first trip across I provided the complete inventory, clearly distinguishing between the things I was bringing on that date, and the things to be brought later.
For later trips the Douanes made it clear they only wanted the items listed that were being brought over in that trip, so I made clear it was a part of the whole: “une partie seulement de l’inventaire complet remis le 22/03/2022”. This was attached behind a copy duly completed of form Cerfa 10070 “DÉCLARATION D’ENTRÉE EN FRANCHISE DE BIENS PERSONNELS EN PROVENANCE DE PAYS TIERS À L’U.E. EXPEMPLAIRE DESTINÉ À L’IMPORTATEUR” available at: Déclaration d’entrée en France en franchise de biens personnels en provenance de pays tiers à l’Union Européenne (Formulaire 10070*03) | service-public.fr
Its a simple form, and apart from Name, Prenom, Address in France, and signing and dating it, it just required a few ticks in the requisite boxes.
Additional necessary documents were: 1) Proof of the sale of the UK property (I used the sale document sent by my solicitor): 2) proof of residence in France, (EDF attestation or similar); 3) Passport; 4) French residency card; and 5) an attestation that you are not going to sell the stuff on arrival (non-cession). I adapted an attestation I found online:
Microsoft Word - ATTESTATION NON-CESSION.doc (atlasintl.com)
You need two copies of all the above for the customs, and should keep one for yourself. I printed all 5 documents above, including passport and Id card, since it made life easier for the customs officer.
Even if you use a removals company you will require all the above, and will have to identify the agent acting for you on the Cerfa form.
I found the Douanes at Calais very quick to respond to email: r-calais-bc@douane.finances.gouv.fr
The annoying thing is that each crossing, they want a fresh set of all the documents (triplicate again), signed and up to date, including the Cerfa form, and the specific inventory for the items in the vehicle. Queuing and being processed takes best part of an hour. However each time I have received a duly stamped copy of the Cerfa form with attached inventory handed back, on which the Douanier has listed all the documents provided.
