I can see no reason not to dial 999?
----------
Agree with that. If innocent.
Doubt that Bamber thought phoning Chelmsford would arouse suspision. He believed it was an 'open & shut' case.
You seem to forget he is trying not to get caught, if guilty he would definitely have dialled 999 why not?
How can he assume it was a open and shut case, lots of things could have given him away, even walking / cycling with no lights he might still have been seen.