For those following along, this is now sorted, thanks to @MughundhanRaveendran-MSFT for help.
Although the original question was asked with a Service Bus, I am now using a Storage Queue.
The problem was that my trigger parameter (the thing I couldn't find any explanation of!) needs to be "connection".
To work this out I needed to read up on KEDA triggers - https://keda.sh/docs/2.7/scalers/azure-storage-queue/#authentication-parameters.