Bramley,
Having built a couple of these myself, and I can assure you, your build is just fine.
They are a real nice engine, smooth running, good speed control, and plenty powerful enough for a model up to about 36" to 40" (not with too much displacement though).
I have a V4 version in my shop built by someone else, that I use to show people how NOT to build a steam engine.
Peter Arnot's boilers also work rather well, so don't have any worries on that score.
Very nice build.
Bogs