s 8 on my model tells me boiler is in anti cycling mode, (saving energy cost ) check the display window and see if you see a flashing radiator symbol FLASHING this NORMAL on timed setting CH settings.
mine does approx 20 mins ON approx 9 mins OFF(anti cycling mode boiler then refires up this continues on /off till the turn off time on the clock
down load yourself a manual from valiant
wirecutterman
June 2010