Thanks for the hint re IKEView, looks to be a really good tool and has certainly illuminated the path!
From IKEView we can see the following:
Site A GW - Local host ==> Remote peer(Public IP)
However from the Site B GW IKE log we're seeing: Local host ==> Remote peer(192.168.xx.xx) where 192.168.xx.xx is the private IP of the Site A GW
It looks to me like the main/management IP of the Site A GW is being presented as the peering IP, however, we've explicitly set this to be the WAN IP of Site A GW as per the below:
We did wonder if we should remove the Site A GW from the SMS and re-add with it's main WAN IP? But equally, would expect the above to take care of the peering element?
Many thanks for the help so far!