Randomly came across a mention of Saethryd, the daughter of
Gyles Brandreth, former
Tory MP & TV/radio presenter. It's Old English - she's named after a 7th-century saint:
http://www.behindthename.com/submit/name/saethrythand is one of three siblings:
Benet Xan
Saethryd
Charity&
Aphra Kendal Alice, named after
Aphra Behn:
https://en.wikipedia.org/wiki/Aphra_Behn