@ben well that's extremely kind of you - and you've embarrassed me so much by responding to my #lazyweb request that I've put some effort in to figuring it out and it works now.
Basically just hadn't quite twigged you have to set up a request payload first, which automagically gets picked up by the http- request node.
I don't think there's any NodeRED integrations for the new pihole6 REST API yet, so just using simple http-request nodes for now.
Thank you for your service :)