Before you got the new knob could you not turn the oven on at all?
Can you push the knob in and out does it feel springy?
The new knob does at least stay on the cooker but it won't turn beyond the 'S' to switch off and I am reluctant to force it
Ok the thermostats do fail quite a lot on these were it jam up and either wont turn on or turn off. As when you turn the dial to the S (slow cook setting) you just push the knob in and turn it off.
As in your case its not turning off its looking like the thermostat itself is faulty as that's the only thing that can stop the knob from turning.
In this case im afraid the thermostat will need to be replaced and that will cure the fault here.
If you need anymore info just get back to me.