How do i change a three way Motorised valve?

Im 99% sure the valve has packed in ( its 20 years old ) its been a bit intermittent for a while now ( heating not coming on ) tried the lever on the side managed to push it upwards with some resistance got a bit of whirring but never returned to middle positon and the lever has no resistance in any direction ie its loose/floppy Ive got hot water and the pump is working ok so when i change it do i

A : Drain the whole system down 1st or can i get away with turning off valves/Stopcocks if i have them

B : Do you change the whole Valve or just the motorised part on the side ,If they are 2 seperate pieces would it be prudent to change them both anyway because of their age

Ive had quite a bit of experience with plumbing over the years so quite competant changed several parts of my heating system ( Potterton 10/16 Neaterheat Bless it its still going ) so not phased by it its just when i phone up plummbers Merchants would like a better bit of terminology as some tend to be a bit awkward if you dont use the correct lingo

Many Thanks

Adam

Adam
December 2009