Graeme,
on this site, you also had an extra version of f0f that needed to be disabled.
The Bootstrap version selection was set to Bootstrap 3, not 5. I don't think you need it anyway since it is installed by your template.
The cart does not show because it is assigned to a wrong position. You need to change that (I set it to top5, to check it out, but it is obviously not the right place. It was set to top1). I set the layout to 'minicart'.
The template you are using overrides the font family and makes it FontAwesome 5 while FontAwesome 6 is loaded. That is why there is no cart icon.
Seems like I was able to go all the way to 'Place order' in the checkout.