The 5 days is the minimum that insurance companies are allowed to stipulate in their policies ( for theft it's two days ). Some have longer. Regarding refusing a late claim, the insurance company has to be able to prove that the insured has 'caused them harm' by the delay i.e. deliberately leaving the claim long enough to prevent an assessor from making an accurate report. Most websites on the subject I've read say that under normal circumstances the 5 days begins as soon as the insured becomes aware there is a problem. This one appears to be the most definitive.
https://www.village-justice.com/articles/declaration-tardive-sinistre-comment-vous-opposer-votre-assureur,43074.html
I think, being the other side of the Channel at the time, Mr Banana won't have any problems. If he does, it's time to consider changing insurers.