Concrete Block wall

[V8]

Hi I want to build a perimiter wall in H blocks, is there a maximum height to build to in single skin? The wall will be approx 12 foot long and 6 foot high and have a 3 foot spur running off at 90 degrees. the wall will be on a poured base. Would filling cavitys with concrete make it more stable or less due to weight?

This is one of those jobs I have been putting off, but other half now leaning harder on me. Suppose this could be classed as preperation so should keep her happy.