Karl, you need to build a base of quality speed endurance. Try sessions such as 100/200/250/200/150/100 with sufficient recovery to allow you to run at 90%. Run the reps even paced as well, so that you maintain speed through the line (not slow down - this will help your 400m running). Do this type of session two to three times a week.

In the next month start to cut down on the reps and increase the speed for two workouts a week an example: 2 x 250m flat out with 20 min recovery between. The other session should involve block work and lots of sprint drills. Watch your energy levels, if you start to slow down in a workout then stop the session. In the week before your comp, ease right down and do some 40's and take a few days off before you run. I can only give you some general tips. Hope this helps.