Transactions of the Bristol and Gloucestershire Archaeological Society for 1877-8', Vol 2, states that Frances (not Margaret) Selwyn married Thomas Hayward (not William Hayward).
This confusion between William and Thomas is relevant because the son, Thomas Hayward, is sometimes referred to as son of Thomas, sometimes referred to as son of William. See his entry and notes for more on that problem.
In one respect we know that this source is wrong - it states that
Margaret died unmarried but we have the marriage records for Margaret Selwyn and William Hayward, so we are sure that the marriage did in fact occur.
Secondly, I can find no trace of a Frances Selwyn, let alone one who married Thomas Hayward. And I can find no trace of a Thomas Hayward who married a Frances Selwyn. So I think this document is mistaken in terms of this point and can be excluded.
The record is also available at
https://archive.org/details/transactionsofbr2187bris/page/280/mode/2up