Hi @az,
Welcome to the Bunnings Workshop community, it is fantastic to have you with us.
You might like to check out the Fixing to Hebel Guide, which offers a number of solutions for a variety of scenarios.
Assuming the flashing is just sitting over the Hebel and not really applying a vertical load that would be carried by the screws, then you could just screw it to the Hebel using these 14g x 50mm Galvanised Hex Head With Seal Type 17 Timber Screws.
Let me know if you have further questions.
Jacob
