Hello from Canada.
The reason I wanted to join your forum is because I have information about Jan Plichta.
Esther Weeks (Clark) was the head WEMYSS decorator at Bovey a Pottery after Joseph died. She knew your Jan Pkichta, the wholesaler, and she is still alive although I do not know if she is still well. Griselda Hill of Griselda Hill Pottery in Fyfe knows her.
There is a copy of a letter from Jan Plichta to Esther in Brian Adam's Bovey a Pottery book. When Esther worked with Brian making Exon WEMYSS, he helped himself to some of her Bovey things and included her name on the first printing of the Bovey book without her permission. She parted ways with him and taught Griselda Hill, who was already bringing back WEMYSS, the WEMYSS painting techniques. Griselda a Hill owns the rights to use the WEMYSS name on her pottery .
I hope this is useful.
Wanda