How to replace the air pressure switch on a Potterton Suprima 80L boiler?

The air pressure switch on my boiler is behaving erratically. A couple of times recently the boiler has failed to switch on (fan fails to start up and blowing and sucking gently up the plastic hose linking fan to air pressor sensor eventually clears the problem). The boiler is 7 years old and has been regularly serviced. What is involved in changing the switch? Can it be done by a competent DIY'er?