Well then. Barring the explanation that John Preston had somehow died in his sleep. Here they were indeed.
Would Errol still be happy if he learnt that John just stood by and watched Mary die in fear as she was incinerated? For the first time, Preston had to decide between confessing everything that had happened since Errol's death - there's nothing happy or obnoxiously wonderful about the truth - or if he'd perpetuate a lie (at least for a little while) to... to maintain a dead man's happiness.
"I..." his gaze flickered. When he answered with the neutral expression Errol would have grown accustomed to, it was quite clear he'd resolved to do the latter.