Can I change a 3 way valve on a boiler myself?

My boiler is leaking - engineer told me that the 3 way valve needs replacing - part is £100 and fitting is another X amount. He told me that it should be okay for the time being but if it starts leaking even more then i should replace it..

How easy is it to replace a 3 way valve - my diy skills and experience? fitted an ensuite shower room in the past. The combi boiler is a sealed system - I suppose I would need to drain the system - replace the valve - refill using inhibitor??

John
October 2010