my Heatline c28 cuts out at 60 degree ?

Hi
Just wondering if someone is able to point me In the right direction how to resolve this Issue.
My Heatline c28 cuts out when it reaches 60 degree and will not go any higher than 70 degree.
During the time it takes to reach this temperature the boiler switches itself on and off repeatedly.
all else seems to be working fine
Any sensible suggestions would be welcomed.
Many Thanks