I got sipgate.co.uk up and running again without any major issue. As Tesco's voip service no longer exists I even went and bought some credit with sipgate for dial out.
Getting Google Voice and Gizmo 5 back up and running like it was back in the days of Grandcentral was more of a challenge (and I'm not yet happy with the way I've set up Gizmo through FreePBX, but at least I got something working). At first Google Voice wasn't sending calls to Gizmo and then I eventually noticed what appears to have been the problem - unlike Grandcentral, Google Voice won't route to Gizmo if you don't have a "real" phone number located in the US attached to your Voice account. I found this both disappointing (one of Grandcentral's selling points was that you didn't need a real phone) and easily surmountable.