But the small fire doesn't pose much of a 'risk' does it, unless it is on your head or in your pocket. The need to stop the fire growing is covered by the duty to mitigate the spread of fire, and 'precautions' should be put in place regarding this. (Only so far as is needed to protect relevant persons though)
Thinking about enforcement and offences is muddying the waters a little. Someone does not have to be put at harm to enforce anything, an FRS can enforce anything that ensures compliance with the Order. i.e. The need to record the risk assessment is not something that will lead to a life threatening situation, but I can enforce it providing that the legislation requires it in those circumstances.
I think there are two main points;
1: No matter how far down you take your 'likelihood', unless the likelihood is zero then we will generally protect against the fire that "could" happen. Now "should" that fire happen, it presents just as much risk regardless of its likelihood, as its likelihood is irrelevant as it has already happened. It is like the chances of throwing two sixes on a dice when you have already thrown one of them.
2: The legislation and the guidance note to accompany the legislation never specifically mentions reducing the likelihood of a fire. Article 4 looks at reducing the 'risk', and fair enough you can impact the risk a little by dealing with the likelihood, (But '1' above clearly comes in to play) but that is not the same as legislation that specifically requires you to reduce the likelihood.
I would reckon that the fact that a fire had taken place when it was reasonable & practicable to have prevented the fire is enough of a failure or ommission to have placed one or more occupants at risk
At risk of death or serious injury? If a small fire can place people at risk of death or serious injury then there must be many other issues. You would have no chance in court.