f28 and f29?

can u please help me. i am still having problems with my vaillant boiler which the plumber has failed to leave me any instructions for. again it still displays f28 when i try to switch the heating or water on via the dials on the front of the boiler. i have switched the power off at both the socket and on the front of the boiler and have tried turning them back on and turning the dial to turn on the heating and water but the display first shows the degrees then f28 and the pilot light still fails to come on although it does sound like it is attempting to fire up. any further help would be grately appreciated