Hi,
Thank you very much for your help.
Millipede, the family you picked out in the 1881 census are indeed the correct lot from what I can work out. In 1861, William Fraser was in Cornwall, I think. The family can be found in Inverness in 1871 and then Cornwall in 1881. In 1891, they are in Campbeltown, Ayrshire, where William and Isabella eventually died.
It is funny you should mention the Camerons/Camerans. My reason for wanting to see the marriage extract is that William and Isabella's son filled out their death certificates. He gave both William and Isabella's mothers' maiden names as being Isabella Cameron. It's possible, of course, that they both were Isabella Camerons, but I was reluctant to add the names to my tree in case he had accidentally confused the names of his grandmothers (I have previously come across a death extract with incorrect information on it in my tree). I thought that if the mothers' maiden names had been on the marriage extract, I would have been able to double check!