Just got some paint from VW. Hopefully it’ll be better than Halfords’ poor effort, but I don’t know if I need primer or not. If so, what kind? Is all car primer compatible with all car paint?

Some top coats need a basecoat in a specific colour and some wont
I would think the primer would be fine but you’d need to check about base/top coat compatability and if its required for your paint.
I’m no expert by the way, just what I’ve picked up from a mate who does some spraying.