OK Pips, I'll answer what I can (The sim isn't on the PC I'm using at the moment).
a) The sim is being released with just three flyable aircraft correct? Are more planned? How many non flyable aircraft are there?
Four flyables, although the Fokker is a two-fer, so only 3 actual aircraft. There are no more official flyables planned that I know of, but The Community has some surprises in store! from memory there's another 4 or so AI only aircraft, which are but a few minutes of notepad time away from being flyable.
b) How many missions in any one campaign?
There are two Campaigns, how many missions is up to how successful you are (or aren't!).
Does the campaign cater for leave, injury, promotion, medals etc a la RB3D? Are there accurate maps covering the frontline France? Is there any movement/action at ground level?
The Campaign works in the same way as all previous SF series Campaigns. The maps are accurate, as far as I know, with some action at ground level on the frontline (tanks only at the moment, but The Community will also provide more variety, with time), remember this is an air-to-air (lite) sim though.
What variety of missions are available to be flown? Ie is it just air combat, or are there ground missions, recon, bombing etc
Again from memory, the mission types are:
Intercept (the Bosch are often attacking your field as you take off!)
How many aircraft can be in the sky at one time? How big are the dodfights?
Same size as other ThirdWire titles, but again you can tweak them with notepad, and you can also (in Single Missions) select up to 16 aircraft for your side, so up to 24 or so I suppose (stock). Also keep in mind these aircraft are the most highly detailed of the stock aircraft so far, medium detail is about the same as high on SF, for example.
s this a stand alone sim or does it dovetail in with the rest of the Thirdwire products
Both! Totally stand alone, or if you want you can import other era. Some of us already have 1920's, 1930's and WW2 installs of this sim!