"an" final pronunciation
To answer your question, we need a little basic phonetics: A vowel has two main attributes -- how open your mouth is when you are making the sound, and whether the sound is made at the front of the mouth or at the back. For example, FLEE vowel is close and front, FLU vowel is close and back. BAN vowel is open (more open than BET, which is more open than FLEE) and front, START vowel is open and back. The FUN vowel is usually in the mid of the mouth, but not as open as START.

The BAN vowel is more open in some accents in England than in the general American accent. As Stan has found, the vowel in http://www.macmillandictionary.com/pronunciation/british/ban is front and very open, more open than the same vowel in American accent.

Now, regarding your original question what 'more central' means, it means that you open your mouth as wide(open) as the BAN vowel in the audio clip above, but move the tongue position a bit towards the back. As a side note, the standard mandarin accent uses a mid vowel in pinyin ban, while the Taiwanese accent uses a fronter variant (just like the BAN vowel in the audio clip).