I prefer not to repeat name 'expert' errors if I can avoid it. :)
Regarding the Flemish weavers, it's surely true that they popularized the names Jankin, Jenkin and Jonkin. In fact, digging around on the net this morning, I found a record claiming that Flemish settlers arrived in South Wales around 1100, and more in 1300(ish) which might go some way to explaining the popularity of Jenkin in Wales. However, it's also to be remembered that there were people of all origins with
William the Conqueror, and the diminutive suffix -kin probably arrived in the UK with the Normans. In the Welsh sample alone I have Watkin, Wilkin, Tomkin,
Hopkin, Dawkin, Dakin etc. Also,
Jen,
Jan and
Jon are perfectly good medieval English forms of
John. So for the medieval English person, Jankin would not seem like a 'Flemish' name...
As with the Welsh/Irish names one needs access to the native records to get the clearest view. For that reason I am always impressed with and interested in what you have to say on Northern European names. :)