The NotificationURL format is invalid - Server Inframe integration on localhost

I'm working on Sagepay Server Inframe integration, using PHP, on localhost and I've been following the documentation and integration guide, however, I've hit a roadblock.

When I send a request to https://test.sagepay.com/gateway/service/vspserver-register.vsp, I receive the following error: StatusDetail=3011 : The NotificationURL format is invalid.

I appreciate that localhost is not an externally visible page and to try and fix this I've added my IP as a valid IP in the test settings on Sagepay, as I had seen this as a solution to another question on SO, but I'm still hitting the same issue.

Does anyone have any experience integrating this Sagepay payment method in a development environment and if so, how can I solve this problem?

Thanks

Whitelist the IP with Sagepay and then make sure the URL you set includes http or https.