Get some advice from someone who has experience in the field of display flying.
I got one about 5 years ago for the 300.
There are not too many Display Authority Evaluators around, Dennis Kenyon and Keith Thomas being the only two who spring to mind.
You need to practice your routine a fair bit, talk it through with the DAE on the day, and then fly it for him as briefed. A discussion on the content of CAP403 takes place somewhere in the proceedings and then a recommendation is made or not based on what you know and how you fly.
I hope this helps, if you want to know any more let me know.
Gary