I think you need to go back to UCLH and explain the injections don't work for long.
It sounds like something changed, and they probably need to investigate what that is, and devise a treatment to counteract it if possible. This is worth doing even if you weren't interested in erections, because some of these conditions are early indications of other problems which need tackling before they cause a major health event.
There is a condition called Venous Leak where too much blood leaks out of the Corpus Cavernosum (expandable part of the penis) for it to remain erect. If the erection was generated by Alprostadil being injected into the Corpus Cavernosum, then it's going to get washed out by this and cause loss of erection. This can also cause light-headedness because the Alprostadil is washed into your main blood stream much more quickly than normal, pulling down your blood pressure. What you could try is a cock ring which applies pressure around the outside of the base of the penis, not a pump constriction ring (which applies enough pressure to close arteries and veins), but something more gentle (such as a Durex Pleasure ring) which just applies pressure to the veins around the edge (although too deep to be seen) which drain the blood from the Corpus Cavernosum. That might help to keep the blood in the Corpus Cavernosum, counteracting Venous Leak to some degree. Cock rings mustn't be kept on for more than 30 mins, and never allow yourself to fall asleep with one on.
Another option would be to use the pump to generate an erection (without any extra medications) and a pump constriction ring to maintain it. Again, 30 mins max.
As for combination therapy... Did you get any sense that Cialis had any effect by itself, even if insufficient for what you wanted? That would indicate if you have any working nerves. With no working nerves, there's no point using Cialis for erections. I think only a clinician can advise you in using drug combinations which are excluded in their respective patient information sheets.