Flute Pastil - why don't u invest in a couple of singing lessons, because it sounds as though the real problem is not your ear if you can hear the notes in your head, it's your voice that's the problem. It could really help you if you were trained how to hit the notes properly, and you would become much more confident in the aural sections.
More generally, here are some tips I have heard about:
- Sight-singing: remember intervals from popular songs (e.g. rising 6th - my bonnie, 5th, Twinkle Twinkle etc), so when you see an interval, think of the beginning of one of these songs.
- Modulations: hum the key note throughout the examiner's performance of the piece, then compare it with the final chord.
- General 'Q's on period, etc: listen to as much music as you possibly can analytically. This really does improve with age and experience. Learn about the basic genres of each period, and listen out for features that were typical of the period (e.g. if it sounds random with loads of clashes, it's probably 20th century atonal stuff).
BTW, when it comes to singing back the melody, you should have the option to play it back on your instrument, so Fruit Pastil, this might be better for you. Even though I have perfect pitch, I hate singing in front of people so always choose this option, though some examiners don't bother askin!!
I hope this helps,
Pianissimo