In the Abbaye Saint-Paul de Wisques (where you can stay overnight), the stamp is a sticker fitting in the créancial.
The monks put a very nice stamp in my credential in September this year. I felt very privileged to spend the night at the abbey and join them for dinner and low mass in the early morning. Although I am not religious, I was touched by the blessing that I received from the monk who sent me on my way in the morning.
The monks are very welcoming, but I have to say that I was happy to be on the road again after my taste of the austere monastic life. Doubtless I would be in better physical shape if I ate frugal portions like them - and with the resulting weight loss, perhaps my knees would not be in agony after 45 minutes of kneeling through the morning mass. But, it's a pilgrim's life for me... If you're of a similar disposition, I recommend having some supplies in your rucksack when you stay with the monks. If, like me, you pass through the next town on the day that the boulangerie is closed ... oh my.
In the church at Shepherdswell, I got a stamp and bought a lovely cloth badge for my rucksack (with 3M adhesive) bearing the VF symbol. It was nice to find a place that really recognized the pilgrimage. The signage and awareness of the VF is generally less evident the English side of the channel than it is when you reach Calais. The staff at the Holiday Inn in Calais (hey, even pilgrims can use points sometimes) were most enthusiastic about hosting me and other pilgrims.
Once you're in France, every marie will have a stamp and a smile for you.
Couple more tips -
Highly recommend calling the "Camping La Bien Assise" to ask if they have a refuge for pilgrims. I had booked at their hotel, which is lovely but expensive. And when I got there, I saw an attractive stone building with "REFUGE" and a VF symbol on it. Perhaps it's in the guidebooks ... I tend to travel without guidebooks.
In Therouane, allow time to explore the heritage center which has an excellent exhibition of the cathedral that once dominated the area (razed by Charles V of Spain) and the pilgrims who passed through. There is a delightful looking pilgrim accommodation in the town.
I found the French northerners (nickname: Schtis) to be some of the friendliest people. I don't think a day passed when I didn't enjoy a chat with someone. Once or twice I had long chats with people who also liked walking, or just wanted to tell me what life was like when the mines were still open and so on and so forth. Just wonderful and I can't wait to get back on the VF. I wrote a few blog posts, which aim to be the least factual and the most accurate accounts of my experiences (apologies to Hunter Thompson for stealing that phrase):
Posts about 2019 Canterbury to Bruay-la-Buissière written by Jonathan
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I didn't bother getting a stamp in Dover. Perhaps I was feeling resentful toward the hostel there, which reeks of mold, dirty linen on the floor, etc.