It seems like they were given every opportunity to sort out the problem and just didn't care

so I'd say the fine is fair enough. It would be different if it was a problem they had been trying to solve but had been unsuccessful.
Our landlord is pretty sh*t, they never do anything. Some of the frame of the front living room window has weathered away outside... Told them about it in October, they haven't bothered to fix it. Also the stairs carpet is ancient and horrible and they won't replace it, even though it would cost a couple of hundred at most... and I'm sure the whole downstairs would be the same - but we paid for new flooring for the whole downstairs out of our own pocket 3 years ago. We also paid to have a horrible 70's gas fire and surround removed (turned out to be a lovely open fireplace behind it!) and also for a modern gas cooker in the kitchen because the one that was there was disgusting and rated something like K for efficiency... But technically worked and was "safe" so they wouldn't replace.
Meh.
That said... I don't really want to rock the boat with them for a couple of reasons. Their lack of giving a **** about the property is what has given us open permission to decorate etc. as we please (we have it in writing that they don't care

), we've literally never seen them face to face which makes the house feel more like "our home", and also they haven't increased the rent since we moved in 4 and a half years ago. We're paying over £100/month less than ANY other similarly sized property in the village... So swings and roundabouts I guess

.
Ill probably end up paying for the hall and stairs to be recarpeted and probably bathroom flooring too (though might have a crack at that myself first

) but it still feels like an OK ish tradeoff.