Thanks for the replies everyone. I feel for your Torana XU1 on those rocky bits of the trail!
I'm just writing a short trip report here in case anyone else ever decides to go here.
I drove towards Mt Tootie and spoke to an owner just inside the Itchenstoke gates. He said he didn't mind but wasn't too sure about the residents of Little Tootie, as that lot had been recently subdivided. He said that if I parked outside the Itchenstoke gate and rode my MTB to the firetrail and greeted anyone along the way, it should be fine. I met several people on the short cycle from the Itchenstoke gate to the start of the fire trail, none of whom questioned my presence.
The firetrail to Bowen Hill and beyond is nice. It extends further than the topo map suggests, as is visible on satellite view on your mapping service of choice. The firetrail is very smooth and a little sandy, with the occasional steep rocky section I couldn't ride my bike down\up (although I'm sure a more skilled biker could). I'd consider it mostly single track riding. There was a larger range of biomes than I expected for a ridge, with some nice wildflowers, thickly wooded sections and lush and ferny parts. The trail is too overgrown for there to be much to look at on the cycle in. It took me 3.5 hours to ride to the end of the firetrail at a relaxed pace in gale force winds. Here is a view from the end of the firetrail:
Beyond the end of the firetrail it'd be a few kms walk along a fairly standard Colo ridge to the top of Pass 37, which would take you down to the Colo-Wollongambe junction. I decided not to do this in the gale force wind on the day I was out. Although it certainly looks like something you could do as an overnighter, ride in with packs and hike down to the Colo, camp and return the next day (or better yet spend a day exploring the gorge!)
Here's an extract from Brian Corlis' excellent book to help the intrepid explorer
Cheers,
Richard