have flushed and checked though coolent  ,reset heater control servo,s not very hot up front and rears are blowing cool air ? question 1 flush heater matrix next ,
              question 2 has somthing come adrift in centre console or under arm rest than controls heat to rear vents ?

I had the same problems.  I flushed just the matrix which was a right pain on 2.0 with big hands which fixed my problem but flushed it through both in pipe and out pipe using engine degreaser (thats all i had) but worked a treat.