A few more thoughts on possible marriage of William's parents:
Baby William's surname in church register was Higginbotham. Presumably his mother expected/hoped to marry his father.
There is no such implication. The priest simply recorded the father's name.
Was it usual for an illegitimate baby to be recorded in a baptism register with father's surname rather than mother's surname?
Not unusual. For Catholics. In Ireland.
In England, my understanding is that things were rather different. The legal position, of course, was that a bastard had no legal father. "
Filius nullis" is the legal term. But my experience is that Irish Catholic registers name the father, where known. Of course, many registers don't give any child an explicit surname at all, which creates ambiguity.
In Ireland, bastards were typically know by their reputed father's name. In life, that is, not just in the parish registers. I stand to be corrected, but I believe the opposite was true in England? It would also be interesting to study if the introduction of civil registration in 1864 changed this practice - the reputed father not being named at all, on civil registration records, by regulation.
