Hi Elinor,
Have just emailed "my"Alexander's marraige cert,which I hope you get ok.
The 1841 Scotish census has some interesting stuff on it for your family.It has Charles as Chas and his occupation as Wak St ,whatever that means.Janet spelt with 2ns.They also have another sister Georgina born 1831.
I agree that the eldest Liz is born unusualy long time after the parents marriage.Jean Mustards dad is also called Alexander and their is a Scottish tradition to call their 2nd son after their maternal g/parents.So it is "possible" that he is a second son."My" Alex first son is called Charles.
Beware of 1841 Census rounds for age's after 16yrs old .They are rounded down ,so Charles could be anywhere between 45-49 years old.
My Alexander could have been off working anywhere by his 16th birthday ,so proving difficult .
Please let me know if I can help futher and if I do make a breakthough will keep you posted.
All the best,Ian