I have counted all names containing an ap and also triple or quadruple names which don't, but which are obvious patronyms eg Davyd John Gwylym Gryffyth. I could have counted adjectival names such as John Vaure and patronymicals where the ap had disappeared but the apparent 'surname' probably changed in each generation, I chose not to do so. The map therefore underestimates the amount of Welshness in 16C Herefordshire, where even occupational surnames may hide a Welshman.
Three new hundreds had been formed by the Act of Union from lordships transferred to the county from the March of Wales. Ewias Lacy was very Welsh as was most of the new hundred of Huntington, Wigmore however was very English with only a handful of small Welsh districts around Presteigne.
A puzzle is the rarity of patronyms in that part of the old cantref of Ergyng covered by Wormilow hundred. There are many Welsh placenames in the area and Welsh surnames were common. Perhaps language shift had already occurred or there was some other cultural factor at work.
The isolated parishes in the east of the county surely shows the impact of recent or even temporary settlers rather than any long term survival. The same must be true of the road into Hereford from the west and lying north of the Wye. In Hereford City itself the muster roll for 1539 had 11% Welsh patronyms rising to 14% in 1542. Clearly the city was a magnet for Welsh people in a way that other towns, Leominster for example, were not.
My base map shows modern parish boundaries and these were sometimes different from the townships used in the 16C. For example there were parts of Eardisley and Vowchurch which were quite Welsh and others which were more English, my map doesn't show these minor variations.
It's easy to spot that the Teme and Lugg valleys were open to the influence of very English areas in North Herefordshire, the Wye valley less so. This would have had an impact on subsequent language shift in Radnorshire.