My problem was that it cooked on a Launch, but failed on a Push. I still dont understand why that was the case. My issue is that this cook failure is not reported back to the developer and hence it is a bit of experimentation to get the cause.
I used snapshots (Revision Control), that I was able to go back to a working(ish) version, then experiment. I would delete all verse devices, test in all conditions, then if the error perisisted delete all materials etc, etc. Essentially it was a game of trying to isolate the down to the issue. Once I thought I knew the issue - I would revert back to that working version and then try the specific solution from there. I had no idea about HLODs until informed and it just happened to isolate the actual issue for me which was Data Layers.
It was’nt easy nor quick to get a solution unfortunately and this is where extra cooking failure debugging info from Epic would really help.