Hi Rosie i haven't checked many tree's as the one's i did look at only had the same 1851 census for Catherine as i did and no other information. I think i've found a possible 1871 census for her and she's living in Linthorpe which was classed as a district of Stockton so that death looks like a good possible for her. I haven't checked that marriage so i will have a look now, i find looking in Scotland a lot harder then other places for some reason.

P.s
Just checked out a few tree's after i played around with giving Catherine the Thomson name and there's a few tree's that popped up they all have the 3 children i have. They don't have anything on my William but do have information on the kids they stem from!
The tree's don't seem to have any censuses and so on attached to them which is making me a bit nervous as to where they got the info from. Some tree's even have the children from the 1841 Scottish census on there tree's but don't show where they where in 1851 when Catherine and the other 3 kids where living with Colingwood?