IR1
If I understand you correctly (I don't use Jepps but the concept is obviously the same) the answer to
"eg. weather at essendon is bkn at 500'. because alternate minima is 910' you need an alternate, or is that 910' the minima if you were using essendon as an alternate.."
is "both".
You cannot nominate as an alternate that which itself requires an alternate.
SO:
1) You want to go to Essendon.
You cannot go there without an alternate, if the forcast is as you describe (BKN005).
2) You want to go somewhere nearby, but use Essendon as an alternate.
You cannot do so if the forecast is as you describe (BKN005)
Now let me just reiterate what Chief Wiggum has said above:
When you are in an exam room at a desk, and you are answering an exam question about this stuff, the correct answer is as per the book (obviously).
HOWEVER
When you are at work, briefing and preparing for a flight, whether you:
load that fuel, track via that point, carry that extra weight, go without this/that item serviceable (etc.) needs to be considered much more carefully. "The book says I can go" without further consideration,may very well get you
.
Know what's legal, but learn what's safe - they are not the same thing. Older, more experienced pilot's are very good for this sort of thing.
Happy IFRing!
CR.