Is it possible to use a vented mini instantaneous water heater instead of a five-litre under-sink tank?

What does ‘any position’ mean in the case of instantaneous water heaters?

With a low-pressure fitting can the third connecting hose be ‘blanked off’ so that a closed outlet connection can be made?

When does the electricity supplier require to be approved?

In the case of a water heater rated ‘21 kW - 400 V 3~ 30 A fixed connection’ does each phase have to have a 30 A fuse?

When is a relief valve needed with a tank installation?