I don't think you can access the Stores Cavern from the canal tunnels. I visited the Zoo in October last year and took these ( sorry - poor quality - phone camera only)
This is as close to the entrance by the chairlift that I could get under the watchful eye of the zoo staff
Down the steps on the right is a gated entrance. The damp and musty draft emanating from it a dead give away as to what lies beyond!
Further along, back past the main zoo entrance is the 'Bat Cave'
Behind the info board it is just possible to see another gated entrance (This was the main entrance to the loading bay)
Sorry its really hard to see in the photo.
Further along again, opposite the Camels, in a wooded area you'll find this 'L' shaped construction.
This is the third entrance to the Stores Cavern. The end wall that is now bricked up used to house a large fan.
The Left hand 'branch of the structure
Close up of the left hand 'branch' of the structure
That damp musty draft again a give away as to what lies beyond.
Just outside the zoo grounds, this looks to me like it was likely to have originally been part of the shadow factory. The loading dock below the red and white barrier is in alignment with the 'Bat cave' which, as mentioned earlier was the main access for deliveries/ goods out.
As far as I know these are the only entrances to the Stores cavern. There was talk of a route through from another cavern, but I don't think this exists. Maybe someone can prove me wrong!
If I find out more, or get hold of the 1970's photos, I'll let you know
Cheers
Dragon's Lair