..because he needed the extra time to clean himself up.
If he was staging calls in a way that depended on establishing times, he would not have delayed at all. He would have rushed back by bike - as Adam says - and made his call straight-away. He then explains his delay in whatever way he likes - messing around with the phone book, etc.
On the other hand, if he wasn't staging a call, then he had all the time to prepare himself and invent a time for the call from Nevill.
I believe we have established in these discussions that he did not have an answering machine, so he had no technical means to stage a call; ergo, he must have invented the call, so there was no reason for him to construe reasons for a delay. If he was ready at 3.20 a.m. after 'cleaning himself up', etc., then he could ring the police at 3.21 a.m. and say he had just had the call from Nevill, and he could later say it was 3.20 a.m. Or he could make a mistake and say it was 3.25 a.m. and if challenged claim that he just mixed the times up as it was the early hours of the morning.