Tried booting through the aws dashboard.
Also tried creating from the command line api tools.
Checked ssh with -vvv and made sure it was trying the key (note that an old instance with the same key is accessible)...
Checked the system log on AWS console (never seems to finish (stuck on keygen for the host) - despite waiting approx. 60m).
System log last line: ec2: -----END SSH HOST KEY FINGERPRINTS----- ec2: #############################################################
System log also contains errors (however I'm led to believe that the error is irrelevant):
init: console-setup main process (63) terminated with status 1
%Ginit: plymouth-splash main process (228) terminated with status 2
init: plymouth main process (45) killed by SEGV signal