You don't have to check "Use custom pools" again.
Just leave it unchecked and you should be good to go.
Ok, done and now working. I think you are saying that MPH shouldn't be configured as a Custom Pool?
On another matter, after I have done the Optimisation, how do I transfer the values to the cards? If I look at the Overclocking section of the Devices tab, it is not pre-populated with the values that have been derived by the optimisation process which is what I was expecting.
One of your cards should have optimized settings now.