Cool (sorry for the pun!), thanks for the hints.
In the end, the brisket was cooked pretty much perfectly, so I have a feeling that my (mild) panic was unnecessary. Once I get more familiar with the kit, and gain confidence, I'll be a happier man.
Will run it on Sunday for a while, without cooking, and experiment with heat readings in various areas, so I understand exactly what I am dealing with. No doubt, I will then throw some burgers in there - seems a shame to waste the pellets.
I'll also follow up with Jackies suggestion of a blanket, for when things get colder.