Have just discovered a different date for the bap of William:-
24 Aug 1758 son of John PHILLIP in Ort Eynon (sic)
This date, transcribed by FindMyPast, is confirmed by the original image, and eliminates the conflict with the bap of Phillip PHILLIPS, son of John, on 24 May 1757, so William and Phillip were brothers, and there is no need for me to find another John PHILLIP marriage.
I conclude, therefore, that the FreeReg transcription is wrong, and that the parents of 'my' William must be John PHILLIP & Elizabeth RUSSEL who married in Oxwich, 21 Dec 1741.
Is that right?
George