Yes, but its much easier to force the defendant to disprove the case rather than to try & show NPM are at fault...
I don't disagree necessarily, I'm just making sure you understand how it works. As HCAndersen hinted at, what's "easiest" process-wise isn't necessarily best (although it might be), you need to consider whether your proposed action is the best way to mitigate your losses. In this case it may well be.
That confirmation is useful - it seems as though it is not the landowner of the whole car park trying to rent out spaces on JustPark, but instead an individual resident (of Flat 45) who is seemingly seeking to sub-let their space (quite how they thought they could do that when spaces don't appear to be pre-assigned is anyone's guess). So it may be whoever occupies flat 45 who is at fault, rather than the ultimate landowner.
