I'm struggling to understand why two orders made on the same day and have most of the same criteria (same date, different time, both are books, same delivery option and address) are delivered via two different companies...

As I said, the first was by Royal Mail whom I have never had a problem with.

I just have everything delivered to work, which is a factory with a large goods in area that most of the courier companies know, so I'm quite lucky there really.

I really can't believe that Amazon still use HDN. The number of complaints they receive from disgruntled customers must be huge.
I don't think I've ever met anyone who has a good thing to say about HDN.