Hi,
Thanks for the info -- I think it must be St Mary. Looking back I realise I missed out the word 'church': '...lyes at Low Layton Church in the county of Essex'. I doubt it would say this if referring to a Quaker burial ground. Also, there is nothing else to suggest he or his father is a Quaker.
William does refer to his house at Knott's Green 'in the parish of Low Layton', if that helps.
Mike