Crochet query...

Excuse my ignorance here, but am I correct in thinking that Crochet is at the moment not a port or part of the base system (in the same way NanoBSD scripts are)?
That is correct(except that nanobsd does reside in base). Here is the procedure:
pkg install git
git clone /crochet
cd /crochet
./ -b <your board name here>
If your building a board that requires u-boot then crochet will drop out and tell you what port or package needs installing.
For example RaspberryPi2 requires:
pkg install u-boot-rpi2

Raspberry Pi3 requires u-boot-rpi3 and aarch64-binutils. Crochet will prompt you to download them.