Let us know how you get on.
I was looking at those UPS3 PP2 curves the other day for someone else and unless I misunderstood, you said that (with LLH in service) with d.00 set to 20kw and both zones on that you were still getting the boiler target temp of 75C, we now know that the primary flowrate is ~ 22LPM so the secondary flowrate must only have been ~ 7LPM to reduce the rad output to ~20kw. I would be surprised if a secondary head of more than 3M would be required to give a (secondary) flowrate of 22LPM, the UPS3 on PP2 will almost produce that, (it will give 19.5LPM at 2.6M) so looks to me like the pump isn't working as it should (or else some fundamental LLH problem), it might be worth removing the pump head and cleaning out the pump ports and also the impeller vanes, a tie wrap is handy for this, if all clean then the pump may have developed a fault, unfortunately this and a lot of other Grundfoss pumps don't display the power in watts from which the performance can easily be derived from the pump curves, you can allways stick a energy monitor on the end of the pump cable. Anyhow, one way or the other, worth bearing in mind.