Lesley, can't be sure whether this is the marriage you are looking for but going on the fact you say they were certainly not together on the 1841 census, they were together on the Isle of Man for the 1851 and 1861 censuses, and you mention they may have been married between 1841 and 1851 to me it does look like a likely candidate. Also, you mentioned John Bates came from Lancashire the marriage from freebmd took place within the West Derby registration district in 1847.

You may or may not be familiar with ordering certificates, so if you aren't, and I apologise if you are, here is the link you need to order a certificate at GBP 7.00 online - (I had to delete this info, as it was deemed 'naughty language') It is the offficial G R O site for ordering certificates.

The reference number you need is 20 826, West Derby registration district, December 1847, John Bates, but you can look this up on - (I had to delete this also)

I don't have access to census information, but did it say on the Isle of Man 1851 and 1861 whereabouts John Bates came from in Lancashire, may be a clue to whether this marriage is the correct one?

All the best
Ewan