OK so the couple in both 1891 and 1901 South Shields census are the right ones. They have two children, Grace Bell born 1888 and William born 1887.
When I posted about William possibly marrying a Martha ELDER, I based it on the following info:
Jun 1888 from freebmd
Martha Elder
Isabella Ennis
James Hughes
William Jackson
In 1891 there is a William born Wellington, Shropshire married to a Martha.
On the 1891, there is a Martha married to a William Jackson, with two children. The baby is 5 mths old and the daughter, 1 year old. Her name is Edith Elder Jackson - which suggests strongly that her father married Martha ELDER in June 1888.
The details from freebmd will give the two couples on each register entry but will not link them together. That's why you can't rely on freebmd to definitely identify the partners in a marriage. Often you can find a couple on the next census, which is why I found William and Martha.
If you can't find the marriage in a parish register, then to confirm whether this William in 1888 married Martha or Isabella, you may have to send for a certificate - this is the only way to be sure that you have the right couple. You can say ' I only want it if the groom is William Jackson and the bride Isabella Ennis', and they usually don't charge full price if it's the wrong partner.
Alternatively - I just had the thought!- you could email the Local Studies library in South Shields and ask if they could find a marriage registered in the JUne quarter of 1888 for a William Jackson and ask for the details. If you ask nicely they are usually very obliging!
Jan