In reply to Jimmy O:
Depends on what the limiting factor is in your fell running, but last year I broke my 5k pb (okay, not fell running, and still very slow versus a proper runner) and got my best ever results in any fell race I did despite not going for any runs. How? I rode my bike a lot and did some turbo sessions. Beware of abandoning all running though as your legs will turn to jelly when pounding downhill.
For the turbo sessions, ensure you're getting something measurable out of it, be it power gains or lower hr etc.. If you can get your heart rate higher while on the bike that is a big help for most people. For example, it is rare that people can access above 95 percent of max heart rate while on a bike unless they're pretty well trained, while people reach 100 percent while running. If you can get strong enough to access that top 5 percent it's hugely beneficial as it means cycling can then train that 'zone 6' or whatever you want to call it. You know, the 'sprint for the line until you can't see properly' bit. Training that on the turbo is rad.