@BruceB Many thanks for that, I'm still not sure about screwing through, though with the method chosen you've done a good job, if I was to do it that way the only difference I'd do would be to use S/S coach bolts instead of screws.
I generally would have 2 concerns, the first is that whatever you do you are stressing the slate, vertical dead loads and a rotational wind load will stress the slates, the second is about using only 18 roof anchors to support 12 panels, the point loads applied to the roof expecially since you're applying the weight of two panels per rafter (6 verticals, 12 panels) is quite a high loading, and the wind uplift in the bottom corners will also be extreme.
If the calcs came out OK, then fine.
I would dearly like a mounting maunfacturer to stand up and demonstrate how it should be done and back it with the regs. (p.s. Paul, I'm still waiting for that info!)