Just bought my first Subaru (Legacy, R,  07 plate) and actually first Japanese car and i love it, but the trip computer, specifically the average mpg is all over the place. The actual.mpg reading jumps all over the place and average seems really low. Any advice would be great.

Sounds normal for that engine. The actual MPG will rise and drop depending on what gear your in and during accelerating and breaking. The average MPG for those cars is high 20's around town and low 30's on longer runs however you should be able to get near 40mpg on motorways runs at a steady speed.

Have you reset it since you have owned it?