Hi Trish,
I don't live near a LDS library to check out their film but have searched the IGI records on their internet site.
I have found another possible child of George and Catherine.
CATHERINE THOMSON
Christening: 15 APR 1827 Middle Church, Paisley, Renfrew, Scotland
Parents:
Father: GEORGE THOMSON
Mother: CATHERINE
Using the naming pattern I agree with you that George's father was a William.
If the above Catherine is their 3rd child, do you think she would be named after the father or the mother's mother.

I have come up with 3 posible George Thomsons. One may be the George you mention.
1. George-b-30 Apr 1803, Glasgow
Parents; William Thomson and Margaret Miller (submitted entry)
2.George-b-02 Feb 1800, Spott, East Lothian
Parents; William Thomson and Margaret Scott(submitted entry)
3. George-b-21 Mar 1793; c-22 Mar 1793, Row, Dunbarton
Parents; William Thomson and Kathrine McLallan
On FreeCEN ,in the 1841 census I found;
Information removed due to copyright violation. See http://www.rootschat.com/forum/copyright.php for more details
RootsChat must deal with any breach of copyright by its members.
For some time the team of Copyright Editors has been removing breaches of copyright and sending detailed personal messages to the member that had posted the information. Due to the volume of posts and members this is now impractical. Messages in breach will simply be deleted and this notice posted. We apologise for any inconvenience caused but are sure you will appreciate the importance of this issue.The first 4 children seem to match up with the ones that I am searching. I am now searching to see if Samuel and Margaret had the same children.
Or perhaps, George and Catherine died and the children went to live with this couple.
Once I gather enough info to go on, I do plan on trying the ScotlandsPeople site.
It certainly is confusing at times.
Tony