Thanks, Jane. Those questions are what I needed to sort things out.
I don't have the baptism details for John or Elizabeth. On their marriage cert it says full age; I'm taking their ages from the censuses. The 1861 census has JOhn born about 1808/9. (supposition).
On the marriage cert the only address given for John is Keyworth.
John was a stocking maker like his father (fact).
So really the only fact I have about William is he is old enough to be JOhn's father (john born about 1808), is a stockinger, and was alive in 1839 when his son was married. As John came from Keyworth and was living there in 1861 and onwards I'm GUESSING that's where his father lived too.
The only William of the correct sort of age in Keyworth in 1841 is living next door to John so again I'm GUESSING it's his father. The FWK fits in too - but practically everyone was a FWK as far as I can see.
The Sarah was new to me when I got the 1841 census, and I thought although she was 20 years his junior she could have been a second wife. I was also unsure about the relationship of the 9 year old child Mary Wright. However subsequent censuses have all found Mary and Sarah living together - Sarah is head of the household always described as unmarried, not widowed, and Mary is described as her niece. As Mary wasn't a Henson I assume she was a daughter of Sarah's (unknown) sister.
I'm unlikely to ever get to Nottingham, and don't know the area at all so I am reliant on the Internet. John and Elizabeth were married at Wysall CofE church - Elizabeth came from Wysall.
So - you've made me face the fact there's quite a bit of supposition in my thinking. Maybe I was afraid of this, which is why I asked for another opinion.
So any more opnions are more than welcome - my head's spinning now!
Thanks to everyone who's helped so far.