Yes, the engine management was OK in the end, I had the manifold/throttle too low for too long. Had some very long flights with no issues so it was me causing the problem.
That said, I was really enjoying your BF108, and was in the process of trying to do a world tour with it, but I keep running into a problem with ground handling. The slightest amount of wind throws it off the runway. I flew multiple legs through England and Scotland and then went to Iceland via the Faroe Islands but once I got into Iceland and the winds got slightly stronger, I couldn't take off. Even with the toe brake and rudder on max it's still pulling violently to one side.
Just tried again this evening, 10mph winds and the plane reacts like it's in hurricane force gales. It may well be accurate, but it does mean it's nigh impossible to use on long journeys.