You need to read that email from Octopus VERY carefully, the only free energy on offer is any you use above what is your "normal use"
It suits me because having storage batteries I never use any from the grid at those times. So I sold most of my batteries content back to the grid (for a high price) yesterday evening, took full advantage of the 9-10 slot and will be able to last until mid-day at which point I will fill my battery to the top. In the meantime the weak sunshine is still powering my house.
I still simply cannot fathom why solar panels and battery storage are not legally required on all new builds (where practical) I have both and I am paying £20 per month for gas and elctric. I am just under £400 in credit, and that doesnt take into account the nearly £700 I have had in FIT payments since January. Think how much that would save being generated if every new house had a similar set up.
My solar panels face SE, so not the ideal orientation, but as of 08:40, with the (weak) sun still low in the sky, they are still producing 1.2kw so I am drawing nothing from the grid.