I'm pretty certain he was 'framed' - planted evidence? But I'm not entirely sure that means he didn't in fact kill her.
Except, given his bizarre escape plan, was he clever enough to have killed her and dispose of the body without it ever being found?
We'll never know. A more recent 'we'll never know' is the A6 murder. I personally am sure Hanratty was guilty, but was his trial fair? It seems to me that if he hadn't changed his alibi he'd never have been convicted. Many are still convinced - despite modern (ish) DNA evidence - that he was innocent.
Complicated stuff.