Agreed.
If you are lucky you could try replacing just the head (the motor) - normally it's these that fail first, but it's not unheard of for the valves themselves to fail. Take care though, this is mains voltage we're dealing with.
Equally you must make sure you get the right valve - Worcester changed them several times and there are about 5 variations so make sure you get the correct one.
For those reasons, get an engineer to do it!
BunSlinger, September 2009