Yeah that's a lot of BS, we've had the same issues in Scotland with that felt. On slated roofs we don't have battens and it's still an issue.
Just a cop out from them making a shitty product.
I've got a carters one, roughneck ones are also decent.
For slate hooks https://www.burtonroofing.co.uk/black-coated-80mm-spiked-slate-hook-316g.html
You can get them in different sizes too.
I always remember a tax man speaking to my old man years ago and he suggest to tradesman not to mention VAT on quotes but just to include it. Says it puts most people off.