20 mg of Prednisone is a significant amount if taken daily. You wouldn't want to go to 25 mg or more. If you can get by with 10 or 15 mg, then do so. The long term effects can be devastating, even from these amounts if taken for a long time.

If you can get a trial of IVIG, go for it. It would definitely be a better overall and long term option vs steroids. Of course the cost is staggering and I don't know if your NHS would approve, but it can't hurt to try. It appeasr you can justify it's use with your ESR coinciding with your pain and inflammation while taking or not taking the steroids. So it should prove the inflammatory neuropathy, which should be covered with IVIG.

I have taken Cellcept (for a short time---until I got a bad infection). It was actually quite helpful and you may want to give it a try first before the IVIG. But after that, I don't think there is anything left that you haven't tried and then they should look at the IVIG. It's just not logical to put someone on long term steroids, knowing the damage it will cause.