Ecodan Fixed Flow Mode - Exceeding Flow Temp Setting

[BadgerBadger]

Fitted our Ecodan heatpump last week with an FTC7 controller, turned on and all seems to be going well.  I was planning to use the "Fixed Flow" mode to try and bring up the screed/house temperatures gradually, so had set things going with a 20C (lowest I could set) fixed flow.  However it doesn't seem to limiting the flow temperatures and is instead continuing to heat as much as it can, I've been turning it off overnight to keep things gradual.

 

Monitoring my flow/return temperatures, it's got to a point of 24C flow and 21C return, so I'm really not sure what my 20C Fixed Flow setting is doing!  Does anyone have any ideas?  I don't have room thermostats fitted yet if that makes any difference, but I had figured the fixed flow mode would just use the flow/return thermistors