It's been painful but I think I've done it.
As you say the air creation trike isn't too bad. What I didn't realise is just how nose heavy it is. To fix the plummeting glide I added another fuel tank in the aircraft's config file like thus :-
[fuel]
Center1= -2.70, 0.00, -10.80, 10.00, 0.00
fuel_type=1.000000
number_of_tank_selectors=1
electric_pump=0
engine_driven_pump=0
Center2= -2.70, 0.00, -10.80, 10.00, 0.00
fuel_type=1.000000
number_of_tank_selectors=1
electric_pump=0
engine_driven_pump=0
Then in the payload I subtracted the extra fuel from the pilot weight (-60 lbs) and voila the trike handles a lot more realistically. glide in hands off trim 450ft per min. Much better.
Thanks again to everybody who made suggestions.
Dave.