The Air Navigation Order is no help directly, as it doesn't provide a specific definition of aeroplane.
However, it does define ‘Flying machine' as "an aeroplane, a powered lift tilt rotor aircraft, a self-launching motor glider, a helicopter or a gyroplane"
It also defines glider separately, together with definitions for free and captive balloon (though no definition for airship, which is often mentioned in the same regulations that apply to balloons).
Given that these unpowered or lighter than air craft are specifically referred to in the Order, but not included specifically in the 'Flying machine' definition, I'd say that the rules of statutory interpretation would mean that they are not flying machines.
By exclusion, therefore, we deduce that a flying machine is a powered, heavier than air, machine which flies.
Also by exclusion, an aeroplane must be any such flying machine which is not (from the definition) "a powered lift tilt rotor aircraft, a self-launching motor glider, a helicopter or a gyroplane". Having said that, the CPL 400 hour experience requirement relates to "aeroplanes other than self-launching motor gliders and microlight aeroplanes", so for these purposes touring motor gliders are aeroplanes and SLMGs are not.
It would also seem to follow that "aircraft" is the broad category of everything man-made which flies (and then we ask if a hovercraft flies and all the non-lawyers die of boredom).
All that this analysis proves, however, is that the answer to your question lies not in the definition of aeroplane (which includes microlight), but in the definition of "microlight" (because hours on microlight aeroplanes don't count).
The wing loading part of the microlight definition in BEagle's posting has been deleted from the 2005 ANO, so if his stats on MTOW and stalling speed are correct (this is stall at MTOW), then the D9 has become a microlight.
In practice, if the CAA tell you in writing that the D9 is not a microlight and that the hours can count towards your CPL, then it should be bound by that letter.
So, don't buy into the D9 before you hear, in writing, from the CAA.
If you have hovercraft hours, I'd give them a try!
[Standard lawyer's health warning: Not my field of legal expertise, so all this is based solely on reading the ANO. Case law and CAA interpretation, which I haven't read, might change this. This advice is worth exactly what you paid for it.]