I may have found out something about the 1:1 NAT.  Apparently the 1:1 NAT will not even try to work if the destination is not in the ARP table of the Meraki.  And, in my case the destination device didn't seem to fill the ARP table in my short testing window.  After trying another destination device, it started working about 24 hours where it did not initially upon setup.  Very strange it takes so long to update the ARP table. ... View more

That makes a lot of sense, and if they had allocated a /56 or other then I think I would have been able to figure it out and split it in multiple /64 subnets.   For the record, this is in the UK with a BTnet leased line for a business. So I assumed they should know their stuff. I'll go back through their documentation and follow up with them as it's possible I missed something.     EDIT: While I wait for them to get back to me, a thought occurs: is it possible there was a typo in the network mask they sent me? That it should be /48 instead of /64? The address they sent me is the first three groups which matches up with the table in the link provided by alemabrahao above. Whereas the /64 has four groups.   I guess since the zeros have been ommitted it could still be a /64 mask but I found it interesting that they match up like that. ... View more

Absolutely makes sense. As I said, I figured it was worth an ask just incase there was something magical I could do router side.     ... View more

I accepted this as a solution because it helped me firgure out what I needed to do. So thank you!   I reset the third switch to factory settings, so it worked exactly the same as the other two working ones. I set the Meraki port as a Trunk Native VLAN same as the others as you mentioned.   I was then able to config the ports on the switch to be in the correct VLAN. This worked fine.   I think the issue was caused because I set a static IP address on the third switch in the VLAN I wanted it to be in. By putting the switch in the Main VLAN with everything else, it worked as expected.     Thank you all very much for your help. I'm learning something new every day! ... View more