so found a hole in the floor of my car today on the passenger side...it is fairly bad....there is a picture below... it looks like rust but it is infact the tarmac below (inside the red circle). The sill and the main floor on the rest of the car all seems very solid.... so for some strange reason its completely rotted in this one area on the car....I have no idea why but the good news is that the rest is all ok! my question is

1. is it a safety issue ? as the sill and rest the floor ok... the hole is located between sill and the floor so not sure if its a structural issue?
2. if its not a safety issue should i just fill it with fibreglass or get it welded ?

MOT 3 months away so good job i spotted it now !