Thanks for the photos, Sally, and well done with mastering the technology for attaching them!
Unfortunately your shot of the back of the dish/plate isn't clear enough to see what this mark is. If you can get a clear close-up I might be able to identify it for you.
The plate itself is typical of the Dutch Delft style produced in the early 20th century, I would guess 1930s judging by the somewhat Art Deco design of the border, and of course the subject matter which is a typical Netherlands scene of working sailing barges and a windmill that became so popular at that time. It's all hand painted (rather than printed) in cobalt blue on white tin-glazed earthenware in the traditional Delft manner that's been done this way since the 16th century when blue-and-white Delftware first became popular.
It's a lovely peaceful scenic plate, unfortunately not especially valuable as these were made in huge quantities at the time and by almost as many individual artists, virtually none of which is a collected 'name' and many examples of these plates still survive.
If you saw yours for sale in an antique shop it would have a price tag of about £75.00. This is also the replacement value for insurance purposes.
I do hope this helps!
As I say, if you can add a sharper photo of the mark, I may be able to tell you a little more. But these marks are usually of curiosity value only as knowing the maker won't alter the value I gave you in this particular instance.