Thanks Mark, yes I thought that marriage was interesting too - shame I couldn't find any other records that might relate to either of them. They were both widowed, but can't see any suitable burials for spouses; can't find any baptisms of possible children; can't see a burial for Elizabeth SYKES (SOMERLIN).
You could be right about the Levett COLLIN. I haven't had much luck with Elizabeth BROWN’s parents - on her marriage it says father John BROWN (labourer), but there are a couple of Elizabeth's born around the right time in Downham with father John, and neither of the mother's had maiden name COLLIN. I think I left it in a too hard basket when I did my initial research, maybe time to get back to it!
And no, I don't think I have any records that say Thomas was born in Witcham, his children were baptised there, but he could have come from anywhere - the 1841 census says born in county, ie Cambridgeshire.