Sorry for not replying, John - I am an infrequent visitor.
The track must have been extended a good deal since then (2000), as I would estimate it is currently about a 1/4 mile in length and winds around a fair bit. Some of it is dual gauge, as you mention - the 'circle' mentioned in the Ian Allan book. But the extension is all single gauge. I didn't measure it, so can't be sure what it actually is, though I THINK it is probably 7".
I might well take another look this coming July, the holiday period, and I'll check it out then.