I thought finding their marriage would be easy seeing as the date is on Elizabeth's birth cert but I just had a thought it may have been a Catholic ceremony!! Which is why I haven't found it and it was 1854 just before civil registration began in Scotland!

Edit to say: I have now downloaded the children's baptsims except William and Thomas can't find them. And indeed most of the births on the census are the places they were born!!! John McMillan the father's trade goes from Pedlar, Coal Labourer, General Labourer, Travelling Besom Maker, Coalpit Worker. The date for John and Mary's marriage is recorded on the baptisms and it ranges from October 1852, August 21st 1853, August 1854 ...

I can't find those census on Scotland's People to check the images!