Flame Failure Light on this beastie means you have a problem and if it contues to be lit after several attempts at restart it isn't transient. It will not be a DIY job. I'll bet the user manual says "Contact Your Installer Or Service Engineer"
Could be a simple 'flame failure fault' - lazy gas valve, faulty ignition/flame sensing electrode etc etc
But could also be a flue fault, if there's a break in the flue the combustion gases mix with the return air within the flue - the flames will start to search for oxygen and lift off the burner/electrode.
DIY how to tutorial projects and guides - Did you know we have a DIY Projects section? Well, if no, then we certainly do! Within this area of our site have literally hundreds of how-to guides and tutorials that cover a huge range of home improvement tasks. Each page also comes with pictures and a video to make completing those jobs even easier!