Dave can you give this Islander FDE a try.
http://www.dmflightsim.co.uk/CBFS_BN-2A_3.zip
(if you downloaded before 14:30, download again - forgot the aircraft.cfg electrics section)
It was further on than I remember.
I'd already took the decision to replace the original Islander FDE with the alternative Trislander FDE (by FSAviator) and take it from there.
Just jiggled it about a bit more & reckon it's not too bad.
Don't know what it will do range wise, or what altitude it can reach, but takeoff, landing & general flying about seems reasonable to me.
It can do 500 fpm climb fully loaded & it can do 230kt cruise at sensible power.
Most importantly increasing power increases VS, reducing power reduces VS.
I've been doing the approach at about 80kt. Not sure if that's right - if not it may need a small change to the flaps.
Anyway, let's see what you think.
Hopefully it can now be fine tuned just by altering the [flight_tuning] section in aircraft.cfg
DM