About two weeks ago, I noticed while on my afternoon run, a group of about four men of possible eastern-european origin, harvesting the cockles and mussels in West Bay, towards the end of the promenade there. Unusual I thought and promptly forgot about it until I heard that others have seen this too.
I have just had a chat with the Thanet coastal project - see sidebar link - and apparently this has been reported elsewhere. It is not illegal but the project wishes to measure any environmental impact and has asked that if you see this happening, please report it to firstname.lastname@example.org
Harvesting of this kind only becomes an issue when the product is used for commercial consumption from a food safety perspective and so there's an equal interest in knowing who the cockles and mussels are being sold to.