It is plumbed in 28mm, with a run of 10' and a head of 8'. The problem is
to
get hot water out of the tank at a sufficient speed.

What sort of speed do you need? I had 22mm to the bottom of the tank and
then no flange, just a run of 22mm to the pump from the top fitting. It
filled a bath in not much over a minute. Are you sure it isn't just a slow
pump? What sort of flow rate do you get? Do you get air in the water/pump?

Christian.