Hi Alfy
I think Pam's census entries seem to be correct ones from the info you have

These are the children showing on IGI at
www.familysearch.org to a Thomas Campbell and Elizabeth (her maiden name is also Campbell):
1. JOHN CAMPBELL Birth: 27 JUL 1856 Blythswood, Glasgow
2. ELIZABETH CAMPBELL Birth: 30 OCT 1861 Bridgeton, Glasgow
3. DAVID CAMPBELL Birth: 18 DEC 1863 Bridgeton, Glasgow
4. JAMES CAMPBELL Birth: 22 APR 1866 Calton, Glasgow
5. ARCHIBALD CAMPBELL Birth: 07 AUG 1870 Bridgeton, Glasgow
The birth certificates are available to view on line at
www.scotlandspeople.gov.ukI cannot see a marriage for Thomas and Elizabeth on IGI and the only one showing on Scotlands People is in 1890 in Glasgow City which is too late you would think. If you check one of the childrens' birth certs. post 1862 (James is the most relevant) it should show place and date of marriage.
Given the birth of John in 1856, Thomas and Elizabeth may have married before the start of official registration in Scotland in 1855 which might explain why their marriage doesn't show on the statutory register after this date. The birth cert of James will hopefully help with this.
Monica