Potterton Puma 80e - Hot Water but not heating?

There is hot water coming from the boiler; this has led me to believe that the pump is working and able to pump hot water OK. When I turn the heating on, there is a whining noise coming from the pump area as if it is getting stuck. I have drained the system several times which has been a cure in the past; however this time it does not seem to have cured the fault.

Any help would be gratefully received