You are right! The problem seems to be with the mid position actuator sticking. It is very stiff! I will pop out in the morning and get some lubricant. Ill keep you posted!
For the last few weeks I noticed that my central heating and hot water was staying on even though my programmer was saying it was off. I checked everything and the only way to turn the boiler off was at the mains! The programmer in question was a Potterton EP2002 and was approx 12 years old...