You don't need to empty the system, Abz, it's a simple job.
Turn off the power to the boiler.
If your boiler still has the plastic cover below it, secured by 4 screws, remove it.
Blast off the boiler pressure by opening the pressure relief valve 1/4 turn (it's just under the boiler on the right-hand side, it has a grey cap with an arrow on it to show which way to turn it) until the pressure gauge on the boiler reads zero.
Remove the power connector from the end of the original valve by moving the plastic holding clip out with a finger, while pulling out the connector with the other hand.
Undo both 22mm pipe connections at the left-hand side and top-side of the valve,(there will be some water from one or both of these connections, but not a lot, so hold a small container underneath to catch the water)
You will see a metal spring clip on the back connection ( to the Dhw heat exchanger) behind the top 22mm connection. Using a small screwdriver (or similar), lift it up and off. You should have a new one in the replacement diverter valve kit, but you can re-use this one.
Remove the old valve by twisting it to and fro, while pulling it towards you.
Fit the new valve by reversing this procedure.
Make sure you have the metal spring clip located correctly, and use the new fibre washers for the 22mm connections. You dont need to replace the rubber "O" ring with the one supplied.
Remember to close the pressure relief valve, and refill the system pressure to 1.5 bar.
Scotty
Scott, December 2008
