Ive taken mine apart and found the middle section of the knob was cracked so the knob just rotated around the spindle rather than gripping it.For the benefit of others it was very easy to do and it has saved me a few pounds as well below is a step by step guide .
Open the softener /wash powder door and gently take it out exposing two screws.Unscrew the two screws and also remove the two screws at the back of the washer that hold the lid in place you can then push / slide the lid from front to backwards and take it off.
The knob is hidden behind the circuit board at the top of the inside of the washer,remove the 4-6 screws and gently remove the front panel.UnClip the connector power block and then remove the 3 screws holding the circuit board in place and gently ease it out of its holder you will then see the knob and be able to pull it off its spindle and check its condition.A brand new dial for this model hv7f140 on partmaster website costs £16 .
Don't forget to unplug the power lead prior to opening up the washer.
Paul, April 2014