As a business we deliver the Highfield Level 4 qualifications including HACCP
In our experience where candidates have failed, not that we have had many (excessive trumpet blowing)
Its not because they don't know the subject, in fact its usually because they know the subject too well
This leads them to try and put down everything they know on paper and as a result don't fully answer all the questions they need to.
As you have said the exam is 2.5 hours, sounds a lot, but not a lot when you have 'war & peace' inside your head and you feel you must get it all down
It is largely about exam technique
Every question has a finite number of marks, once you have achieved those everything else you write is just taking time away from other questions.
Read the questions carefully, very carefully, no more carefully than that, every question has a verb that is key; 'describe', 'explain', 'review' etc.
Decide what information is being asked for
provide that information, do not deviate, do not try to put down a lot of extra info, its probably not going to help
If you have time left at the end, and you are still capable of writing then use that time to go back over the questions you have answered and add in anything extra.
But make sure you have answered all the questions you need to first
hope this helps