I have a Vaillant Ecotec Plus combi boiler that was installed just over 6 years ago. It has been serviced annually, the most recent service being about 2 months ago. The controller is a Vaillant VRT 350f wireless type. Recently a spanner symbol and a “Service Heat Generator” message have appeared on the controller screen (but not on the boiler itself). Otherwise everything appears to be working normally. I am guessing this is some sort of counter in the boiler that is intended to indicate when a service is due. How can I reset this (or do whatever is required to remove the message on the controller)?