Hi Graham,having IOS-XE SW 17.18 on the C9500 is not enough. The Meraki network itself must also be on 17.18.As soon as I upgraded the network to 17.18, things changed. The SVL configuration worked, didn't even need the routed port configuration.Hope...

Tried FF, Chrome and Edge

C9500-24Y4Cmy setup is a setup where L3 is done on the MX exclusively. The Switches are L2 only.How is your setup?

and at the port level 

Unfortunately not. I only have the option VLAN.