Yeh if I were you I would call the number and see if you can organise the visit yourself, HR might be really busy and not realise you need the visit sorted sharpish!
I done my online test last night and thought I done really well, but just felt it was a bit weird as it wasnt a NATS website, so I dont even know if NATS get sent your results as how would they know to pass onto NATS??