I'm doing large rides once a fortnight (200k+) but I'm looking to improve my sprint at the same time.

I'm currently doing sprints at the base of a hill (roll down the hill to gain speed and sprint at 100%) and I've done lots of LT training but I strugle with my max speed.

I've got some long races approaching and I'm confident that I've got the endurance to be there at the finish so I'm working on my sprint.

What work would you suggest for a rider with an absense of fast twitch fibres?

Ive been told many times by many old people (riders) that the best thing you can do is to sprint down the actual hill and keep going full on till exhaustion, passing through the pain barrier.
See if you can find my thread on sprinting that i started a while ago. Had some very good info.

_________________
i dont need some witty signature

The issue is that I'm completing an endurance phase and my speed is suffering.

I produced speed via slow hill work in the past but I'm wondering what other riders have done to maintain their speed during endurance training.

Just doing endurance, especially in large volume, will decrease muscle size and power and lower your manly hormone levels so you should always include a few maximal efforts. In mass start races it's not the guy that can average 375watts the whole way that wins, it's the guy that can hide out at 150w the majority of the time and then put out a higher max to make the difference and win. So your training should reflect that as much as possible.

You should always include some sprints and intervals in those 200km rides. Doesn't have to be structured, just blast up small hills or chase cars once in a while. If you are anticipate having to win in a sprint do all the sprint work at the end of your 200km rides so it will duplicate the actual race requirements.