When you book your ticket you will receive a form in a confirmation email that will allow you to book your room(s).
Although the event runs Friday to Sunday, you may want to arrive on the Thurs evening / leave the Monday morning, hence block booking some rooms for that.
At this point you just fill in the form to let the hotel know what type of room you want, and for how many nights. Closer to the event itself, probably a few weeks before, the hotel will contact you to take a payment. So don’t worry if you don’t hear from them. Provided you fill in the form, your room will be reserved.
Note that you have the option of booking a room with ‘Dinner’ included, for an extra charge of £10-£15 per person per night (the charge is £5 lower per person if you book double or triple rooms). This will give you access to the a la carte menu in the main dining hall. Otherwise you can just order bar food. It might be possible to access the main dining hall on an ad hoc basis if they have capacity, but no guarantees. So that’s what that option is referring to.
Please note that the room booking arrangements are not the responsibility of the organisers of PunchedCON. The refund policy for the room itself is set by the hotel, and set-out in its T&Cs.