[secant0give]

... or is there an issue with the thrust-line? Or is there a negative angle of incidence on the tailplane (my money is on the latter!).

I tend to agree with Steve, I reckon it's the incidence that's wrong.
If it passes the C of G test.
Wich is as follows :

first trim it out to fly straight and level right side up at 60-75% throttle, Then once satisfied with that, half roll over to inverted and take note of how much down elevator you need to hold her level in pitch.

If you need heaps like 1/2 down elevator stick then it's Way nose heavy
If you need just a little, say less than 1/4 stick then it's probably all right for sports flying and will likely pull out of a verticle dive automaitcaly. (positivly stable in pitch)
If it needs no stick at all to hold it level inverted then it's neutral. Good for 3D and slow areobatics.
If it needs up elevator to fly level then it's tail heavy and you'll notice that it's not so nice to fly, especially at any speed !!

If it passes the C of G test and flies nicely inverted with just a little down elevator and still has a lot of down trim on it then almost certainly its the insodence.

You might be able to fix it by packing up the wing to bring it more into the same plane as the horosontal stab or you may have to change the angle of the tailplane.