UPDATE….
OK rode it on Saturday morning. Its fully ridable. Starts at Kemble airfield. Ends somewhere near the M4, where it turns into a road that continues it direct path into Batheaston.
Its mostly similar to a very wide canal toepath. Utterly straight, and virtually level just a few slight inclines. Surface for the most part a bit dull, the occasional rock. But there are one or two places where it mixes it up a bit, generally where it crosses a ford and the 4x4s have carved it up.
Only one more thing to add, it was utterly deserted. For nearly 40 miles I only came across one motor bike and one quad bike.
So to sum up, not really a mountain biking experience to write home about, but probably the most direct 40miles I've ever riden.
Cheers guys