Ah, an input from Grant technical folks which I will repeat here in case it is of use to anyone (and on the offchance it may be correct 
). I inquired about the ‘setpoint temp correction’ parameter in the heat source setup options, and was told:
“The Set point temp correction is relating to a correction temperature to prevent overheating of a space. If you set this to for instance 2°c it will decrease your target flow temperature by this amount to try to help prevent the space overheating. This is more of a trial and error parameter where small changes make a big difference, my advice would be to leave it where it is if your heating is working okay.”
Not sure that makes sense, when we already have the ‘circuit temp correction’ (which increases target flow temp above the WC calculated value), plus WC curve shift options (which can go either way). It must only apply under some circumstance I haven’t figured, or have some effect beyond just winding the LWT target down a bit (which it doesn’t seem to, when I change it).
Maybe it only applies when pump blockade is off, and the room stat says ‘satisfied’, so this correction is applied as well as the reduction coefficient related to how far over/under you are? I have pump blockade on, so wouldn’t see it.
