Sorry for the late reply, currently on holiday in Switzerland and Italy....
Only the part between Castletown and the MOD is indeed offically open to the public.
From the old Stowe by Chartley station to Chartley Moss is a nice bit that you can walk.
It gives a nice view from the old Chartley castle.
Past Chartley moss there is also a path, but as with all the other paths not officially open to the public.
On the Stafford side you can get down to the tunnel entrance. But the tunnel is severly flooded.
Take high boots (or even better waders) a hard hat, a good torch and an upgrade to your life insurance, before going in......