Hi Duck
There is a William baptised 17.11.1822, son of Willam and Nancy; his father is a butcher in Ribchester, though, not Longridge or Alston - the registers seem to be quite specific about the village or hamlet people come from. The records from Longridge St Lawrence and Stydd Chapel, Alston are included in the Ribchester registers, btw.
The only other William anywhere near that time period is in 13.10.1811, the son of William and Mary, of Hothersall.
I couldn't find them among the marriages either. If you like, I could look for Mary Holden's birth for you when I am back at work next week, as I have the registers on microfiche, and can only look at them on the reader there.
If WIlliam's parents were non-conformists, you may be out of luck, as the county record office told me that they have very few non-conformist records for the area, which is a real nuisance, and I've already tried visiting Knowle Green Baptist chapel, where the secretary was extremely helpful, but the register from its foundation to the 1890s has gone missing.
If you haven't already tried them, I've found that the new online Non-comformist records site has a search facility so wide, even on Advanced, that for a common name like Kay it is of no use, unless you are very rich - at £2.50 per record, it's just not possible to check through the list of 10 or 12 hits you get for each search

There
is a William Kay (he's down as Gulielmum, not William,) born 14.5.1819, baptised 16.5.1819 at Saint Mary and Saint Michael Roman Catholic Church, Alston, son of Elais and Mary - I found that via the IGI, (batch number P017581) as I also have been searching for a Wiliam Kay from around 1820 amongst the children of Elias and Mary whose births I can't trace - Elais is already a bit of a religious flea, having children baptised both at St Wilfrid's and at Mellor Methodist chapel, depending on where he is farming at the time, but becoming a temporary Roman Catholic sounds a bit far-fetched!
I'll be seeing if I can get a look at the original registers from St Mary & St Michael when I am in the record office again at Easter, so I may find out a bit more about this William. Do you have the marriage certificate for William and Mary, and if so, does it give William's father's name?