Happy New Year Mr. B.
It sounds like that you want to get the most out of limited training time. For the past 5 years or so I've maxed out at about 5 or 6 hours training a week (job, kids etc.) and haven't done bad on what some might consider to be too few miles. And that's not just in short BOFRA races too. It has been enough for good results in longer races like Rydal Round or Anni Waltz too. What I lose in time I have to make up in intensity so I don't do anything particularly steady and include a fair amount of interval work either as hill reps or on the turbo. About 2 years ago I found a great book which related pretty well to what I was already doing called "The Time-Crunched Cyclist" by Chris Carmichael. Although it is obviously a cycling book, the principles can easily be transferred to fell running. It is well worth a read. I tweaked my training based on its principles to good effect. With such limited time it acknowledges that there are limits. I wouldn't be able to race something like the Three Peaks well with such limited training, but that's fine. I enjoy running short / medium races. Most people infact do if you look through race results. I would lower the intensity of some sessions if I had more time to train, but that is the point, I don't have more time so I have to use the best of what time I have.
Hope to catch up with you at a race sometime soon
Harry