Follow up
Got this from a very helpful tech at paypoint

It might be useful to list the callback urls. On Cuepower it’s:
http://www.cuepower.co.uk/paypointne...ointnet_hosted

Dartspower:

https://www.dartspower.co.uk/paypoin...ointnet_hosted

So it looks like they’re both following the same protocol. The success and failure options have both been specified (separated by a semi-colon) – my point is that on the cuepower website, it’s returning to the failure url (…&[payment_error=paypointnet_hosted ).

I'd be very grateful for any insight