I have found a birth for a Peter W Allen in Jun Quarter 1920 in the West Derby Registration District, Lancashire (ref 8b 1309). The mother's maiden name was Sweeney.
There was another child born: Mildred H Allen, in March Q 1923 also in W Derby ref 8b 997, where the mother's maiden name was Sweeney.
Peter Allen and Mildred Sweeney were married in Jun Q 1919 in W Derby.
Does this sound right?
Coming forwards, there was a marriage for a Peter W Allen in Sep Q 1946 in the W Cheshire RD (ref 10a 1823), but there seems to be a bit of a hiccup in the GRO index in that there is a note to say the bride's surname was either Fellows or Stapley, first names Edith G. Not a Catherine in sight.
Did Peter William have any children of his own?
I ask that because there were two children born to an Allen / Stapley combination:
Peter M Allen born Mar Q 1945; and Pauline M Allen born Dec 1946; both births registered in W Cheshire.