Vaillant Ecotec plus 415 hot water not as hot as it used to be

Hi

The h/w temp was fine (cylinder stat 60°c) before two additional rads were installed.

I noticed that the header tank was empty (blockage in inlet pipe) and the pump seemed to be making a noise as if running dry.

Now h/w is cooler as I don't need to add c/w to my bath now. The last fault code was F23.

Hope this makes sense as all I can do is relate what I observed.

Any advice appreciated please.