Speccy told me my laptop has 4 slots in total (2 used, 2 free). Turns out it only has 2 in total, both used. I guess I should have looked myself before ordering 2 more modules, because now I have to pay to send them back.
Its too late for fixing this to help me in this instance, but if I can offer the devs any information that will help them fix it for the next time I need it, or for other people, then I'm happy to supply screenshots etc.