It's important to realise that good literacy wasn't very high on the agenda of much of the population in the Middle Ages; survival was what was on their minds! This means that the final spelling of a person's name was often left to the stonemason who prepared their head-stone, and so, whether it was correct, incorrect or simply phonetic, the spelling of the name became carved-in-stone as it were!

In Lancashire there are still the place-names Woolston, Croft, Martinscroft, Wolstenholme etc. It's also important to bear-in-mind that in migrating from say Woolston, to mid-Wales, for the Ws the spelling and pronunciation of their name would have been subject to two completely different local dialects, the latter of course being Welsh.

Also, the "z" is usually absent in early records of the name, "s" being very common, of course.

Celtic scholars will also know that in Welsh a "w" in the centre of a word or phrase is a soft "ou" sound as in Cwm, pronounced "coum", so it's no surprise that in some Welsh villages, for example, a David Wozencroft is known as David Ousencroft or something similar, maybe even with a Z-sound instead of the S.