It is possible to remove and clean the diverter valve assembly. Possibly no replacement is required.
I had one that started to cause a problem with the radiators getting hot when the bath tap was running. This did occur after about 5-6 years in service. It turned out to be full of deposits that could be cleaned off.
TO DO THIS TASK A COMPITENT PERSON WOULD HAVE TO SHUT OFF THE GAS AN ISOLATE THE ELECTRICAL SUPPLY TO THE BOILER. THE UNIT SHOULD BE COOL BEFORE OPENING TO AVOID THE RISK OF SCALDING. Then the under the unit the shut off valves for the water inlet and the heating in and out can be closed.
If the unit is cool, remove the front panel and the 2 screws to allow the control panel to flop down. The diverter valve is bottom left on the base of the casing retained by 2 screws.
The boiler can be drained using the drain valve on the right hand side. Be careful because there is some quantity so be ready to catch it!
Removing the valve involves taking off the micro switch on the front. Detach the two small pressure differential pipes with a 10mm spanner, releasing the two screws behind the retainer for the front diaphragm housing and withdrawing the housing.
The two cap screws on the right hand side can be removed where it is connected to the part that attached to the heat exchanger. The connection to the heating out pipe can be released from underneath the boiler.
Remove the top micro switch further back for access then carefully disconnect the pipe in the back which is attached as push fit with a retaining clip.
Remembering the two retaining screws it should be possible to remove the diverter valve.
The body of the valve itself comes apart by retaining the body and unscrewing the back of the housing.
Probably the valve just needs a good clean so the shaft(s) slide nicely again.
Anon
January 2011