Thanks Bookbox. Yes, I have been checking the hints rather that just blindly accepting them. In fact most of them are links to records I'd already seen years ago, before the facility to link them to your tree on FindMyPast even existed, so it's been quite easy. This one caught me out a bit because previously I only had an approximate DOB for William, calculated from census records and I'd not pursued his birth certificate.
I was aware of the 1851 census record that you pointed out and that Sarah re-married, but I haven't yet got around to properly verifying it all.
I also saw the suggestion on the other birth record that Sarah's maiden name was Collins, and I vaguely remember coming across this previously (I'm not terribly organised with keeping track of unfinished bits of research that never make it into my actual tree), and that I was a bit sceptical, but I suppose it wasn't quite as unusual back then for someone to marry someone with the same surname (or even a cousin) as it is now. I have found at least one other case of a family member having married a distant relative.
Really what had puzzled me was how the mother's maiden name was appearing in a transcription when that information is not on the image linked to that record.
I'm sure FindMyPast never used to show that information - otherwise I'd have found it. Have they gone to all the individual certificates and transcribed the MMN? If so - how wonderful - I just couldn't find anywhere on their website that confirmed that was the case and I wasn't sure if I could trust the information.
I did have a jolly good look at this particular line quite a while ago and thus had found the family in every available census bar the 1851 one, but I do my research is mostly online and a lot of record sets have been added and the indexing improved since I started doing this about 15 years back.
Anyway, I'm quite enjoying re-visiting and reviewing my tree gradually - going over each line in turn, verifying and seeing what new data is available. There are about 700 people on my tree, and I was pretty meticulous with my checking before I added people/information, but most of it was done at least 10 years ago, so there are bound to be lots of gaps I can fill and things to be corrected.
It's like a garden - never 'finished'.
Thanks for your help.