Hi Connor,
Many thanks for your reply, much appreciated!
It's definitely worth testing with other (non-HP ProBook) devices as this is the only way you'll trace which side of the wireless network the problem is with.
The only thing I'd check is what kind of channel settings you have on the Unleashed network. In some instances, when the network is newly installed (or just rebooted) ChannelFly has an agressive channel-change algorithm for the first few hours of the network being online and this can disrupt some clients. Additionally background scanning can also disrupt some clients, so it's worth checking the settings.
If you have static channels configured then you can ignore the above.
I hope that helps!
Darrel.