my indesit washing/dryer machine knob is spinning round and round?

my indesit washing/dryer machine setting knob is spinning around and around when it starts washes.
I commenced a wash on no 7(40 c), the knob span all the way round to no 6 and then washed. i have tried it on others, but it spins round to no 6.
Cant set it on a spin to finish the cycle off.
Set it to 8 earlier and it spun round to 7 and is now washing.
any help would be appreciated.
its an indesit wd10

thanks