Hi @MattD
I don’t visit this group much but the members are so kind and helpful. 
Sorry to hear of your struggles and I still do experience those.
So I explored the ijv compression and eagles syndrome. I ended up having a catheter venogram with ijv ballooning. I had only very subtle gradients and ballooning unfortunately didn’t bring me any temporary symptom relief. So I went back to the csf leak drawing board and further imaging has discovered a spinal csf leak, potentially from a calcified herniated bone/disc. So I believe my pressure issues are in fact from a spinal csf leak, especially as I had an obrupt onset. I was absolutely fine the day before.
I hope you get some answers and correct treatment for your symptoms