Phoned the helpdesk this morning and the guy I spoke to reckons it's all done remotely; just reboot the router and it will pick up the static ip address; does that sound logical?! I was expecting to log into the router and add it manually but he says that's not the way they do it.
Absolutely standard that you don't configure your WAN IP address on your router, regardless of whether it is static, dynamic or CGNAT. Think of the mayhem if punters were setting their own IP addresses.
What might be poor is if the helpdesk can't tell you what they think your IP address should be. Obviously, you can find it from the router if the connection is live and there is a tool on the main part of this site to find the address.