Update 28 May 2017Hi,
A few weeks ago it was suggested to me that I try to get both of Isabella's marriage certificates. I attempted that and managed to get the one for her first marriage.
However I've just had an email from GRO saying they can't find her second one.
It says
"Thank you for your order as detailed below.
Marriage Certificate: Isabella Harwood and Michael Buckely married 01 December 1862 in Caistor. (I know I was given an exact date but if I recall correctly this was the closest the system would let me put but I could be wrong about that)
We have been unable to process your application, please refer to the paragraph below.
We have searched the indexes for events registered in England and Wales during the years specified (1861-1863). We have been unable to find any entry with the details you provided."
Then there is more about how I can look for more info on this and that they have given me a refund.
So I'm not sure what is going on there because it seems fairly clear from the sources you all kindly provided for me that the marriage did occur.
I can only assume that I've done it wrong somehow when I filled in the form wrong, but then from the email they sent I can't see how.