I have some good news for you all.
Alfie's day went well yesterday. The angiography showed that the extra chamber in Alfie's heart was functional and taking blood in. The vets went ahead and put a hole in this chamber to connect it with the right atrium, and then used a balloon catheter to widen this connection. It was all done via catheters and instruments in his groin, so I am relieved he did not need to have open heart surgery. After the surgery was performed, blood appeared to flow around his heart "normally".
They made the decision to wake Alfie up after this without doing anything to the double chambered right atrium, as they felt that a) this is not causing a life threatening obstruction at the moment, and

Alfie had been under anaesthetic long enough - 4 hours.
He recovered well and was returned to Liverpool Small Animal Hospital last night for hospitalisation. I cannot describe how grateful I am to these vets, particularly Dr Jo Dukes-McEwan and Mike Martin. Without their expertise, generosity and care this would not have been possible.
There are still a few risks - Alfie will need to stay on heart medication for the timebeing and will need to be closely monitored for complications. There is also the risk of renal failure caused by the angiography contrast, but he is being cared for well.
The best news is - I may be able to bring him home tomorrow!
(and we've raised the amount we need!)