Can the developers give some guidance on how strategy impacts on the game play as I am finding it increasingly frustrating.

Today I played a league match against a team with an average rating over 7 points lower than my team and lost by nearly 20 points. Without changing anything I get challenged to a training/friendly game against a team with a 3 point rating advantage over me and lose 8-5. Thus cannot be due to day to day variability as games were on same day, strategy has not been altered so please can someone explain the massive differences in performance

Average ratings are misleading. They consider the whole squad not the playing line up. Some people have a lot of low rated players that they haven't got rid of and this brings the overall average down. Look at the opponents first 15 to get an idea of what you are up against.
Strategy does play a role. I just beat a team rated ten points above mine. All you can do is play around with strategy to find what works. Problem for me is that I now have a strategy that is fairly good but not fantastic and I dare not try tweaking it because I don't want it to go the wrong way!