Advice please on how to fix the following problem?

I have a Swift Charisma 2003/4. When the water system is left for several hours it takes some minutes with much gurgling splashing etc for water to be discharged from both kitchen sink and bathroom basin taps - by far the worst at the sink tap end. It is apparent that water has run back into the water container sometimes with the consequence of the water heater fuse blowing. I cannot see where a non-return valve is fitted into the system.
fuse blowing - I assume due to water having run out of the tank. Could the problem be a worn seal at one of the taps or otherwise?