Final follow-up. There was an absolute load of crap came out of the rads after emptying the step 1 solution.
The first 2 flushes with the step 2 were very blue as well, but 3rd flush was much less so. I did do a 4th flush too because I wanted too, then ran through all the rest of the DI water I had (bought about 15 Ltr's....) just as a final flush.
Had it running pump only with Mayhems XT-1 Clear in the loop and the system assembled to do a leak test overnight and all looks good. Do some final cable management this evening and should be back running. Will be keeping a close eye on the loop however to see if there's anything strange happening again.