can`t get hot water

[john6869]

Hi all , I wonder if i could ask for some advice ,I have a worcester 28i junior and for the last couple months the pressure gauge has been jumping around now and again ....i believe this is known as kettling and now my hot water will not run unless the heating is on . Is this the diverter valve or the pump. I would be very grateful for any helpful advice .
many thanks
j.mc

 

[kevplumb]

if it was the pump the heating wouldnt work

 

[john6869]

if it was the pump the heating wouldnt work

thanks for your comments ,but my understanding is the impellar in the pump can be damaged ??
many thanks
j.mc