As I understand it, you won't get into the gig as a pay on the door.
You have to buy a festival ticket and then exchange it for a wristband on the day.
The wristband will get you into any one of the 19 venues hosting Camden Rocks during that day.
The problem is, once a venue is full then it's a "one out = one in" scenario.
The Underworld site says it's a 500 capacity room.
NMA played the Jazz Café (420 capacity) the last time they were there but I've no idea what the crowd, or queues, were like.
There last time I was at Camden Rocks, people mostly moved out once the band(s) had finished as opposed to staying in one place for any period of time.
Having said that, if I was going this year, I would go to the Underworld for the band before NMA, or even two bands earlier. I'd rather get in than leave it to chance. I don't think you'll see them play a smaller room.
A link to Saturday's bands / venues / times:
www.camdenrocksfestival.com/festival-info/stage-timesYou may have seen it but these are pretty comprehensive;
www.camdenrocksfestival.comhttps://en-gb.facebook.com/CamdenRocksFestivalgood luck
I wish I was going
gh (h)