That widget in the corner is Voxelmap mod. It's awesome.
I never play creative either, and the slab was indeed jungle wood although it looks greenish, it's from Bdouble0's texture pack.
I guess I'll try removing a mod one at a time although I can't see how any of the mods I'm using would interfere with that chest not opening.
Thanks very much for your help. Much appreciated.
Courageous, (sorry I can't see the rest of your username) I posted it here because I assumed it was from minecraft itself, not a mod.