Same problem as all y'all, O2 Uk Lumia 720 dev preview only installed, I have no cyan and no official 8.1, on my second attempt now but if it fails it is safe to say it is another screw up in the continual screw up the updates game !
If it succeeds... hazaaa !!
Second attempt failed (0x80188308) .... really !!!!!!!!!!!!! Now what ?
2.10gb free on phone, 361.69mb free on sd card
Just had a thought, my phone was set to install new dl to sd, so now changed to phone ! Its a possibility so trying again