hey @alistairwilliams
please dont email your bug to me and post it on the forum.
usually i dont do email support.
we talked about this via email and I said:
"Hey Alistair
This is because of some changes made in the way lay theme shows the custom phone layout.
For this to work properly you need to disable custom phone layout.
Now if there is a custom phone layout the markup will exist with the desktop phone layout side-by-side.
It seems that the same shopify button cannot exist twice on a page. so then it doesn't work properly.
You can either disable the feature "custom phone layout" in "lay options"
or you can use the old lay theme version and old versions of plugins
if you choose to do that, let me know which lay theme plugins you use"