Another point here is that if a heatbank is direct and is using an
internal tank for the F&E, it muct be installed so that the tank is
above the highest radiator.

About the only constraint.

This can be quite a significant constraint, especially if a loft conversion
is envisaged at some point. However, it can be overcome by having a sealed
pressurised circuit for problem radiators (or even all of them) running off
a pumped plate heat exchanger off a direct heat bank loop.

Christian.