Can someone explain to me the difference between the Aural Specimen Test Book and the Aural Training in Practice book as they both seem very similar aside from cost.

One of my main issues why I didn't get a distinction in my Grade 4 was down to my aural so I want to make sure I do extra training for my Grade 5 so wondering which book(s) would be best to get.

They do what they say on the cover.
One is just test and the other has guidance on ways to prepare for the tests as well as some specimen tests.
To use them on your own possibly with the CDs you will need a reasonable grasp of the theory behind the questions as checking your answers requires looking at the printed score to see if you were right. Some people see this as the books not having the answers in though it's not how I see it since i can see the answers clearly from the score.