Antonia Forest fans

Pam and Geoff Marlow are remarkably fecund; but Helena and Anthony Merrick only have one child. Anthony Merrick seems to be a fairly strict Catholic, so why is it that Patrick is their only child?

I wonder whether Helena Merrick, being a rather pragmatic Catholic (see Patrick's remark about fish on fridays in Attic Term, for example), was secretly using some form of contraception. Or perhaps they were playing Vatican Roulette and were just lucky...

I guess from AF's point of view, Patrick needs to be an only to contrast with the Marlows; and maybe he also represents her: she was also an only child.