Hi James
Thanks for your help. Since posting my original post, another researcher has suggested that James was indeed another son of Archibald Robertson and Janet Moody – but neither of us have found his baptism record so it can only be a supposition. The fact that he shares a grave with Colin Douglas Robertson (grandson of Archibald and Janet) in Liverpool suggests to me that they are related, and this scenario would make Colin James' nephew .
I hadn't pursued Isabella's roots actually, but I do think its likely that she is the daughter of John Hamilton and Isabella Gellie as you suggest. I've just done a trawl on Scotland's People, and she's the only one that comes up in 1803. It would mean that brothers Archibald and James married sisters.
Can I ask if it's the Robertson link you're interested in, or the Hamilton side? If it's the Robertsons, you may be able to help with another little issue...!
Thanks again,
Nicola