However the stain block (water based) is still showing the colour through
after two coats.

As far as I am concerned, no water based stain block can be as
efficient as a spirit based one. A long, long time ago, I was taught to
use shellac knotting to prevent bleed-through, and the only reason I
now use Zinsser is that it is slightly paler, and can be used more
easily under one-coat paints.