Thank you for your enquiry. A Tenancy Agreement is much the same as any other contract in that it imposes certain requirements on the parties to it. In straightforward terms the Landlord has to provide you with a property to live in which is habitable and to which you have the right to quiet enjoyment. In return you agree to pay your rent and to take care of the property whilst you are living in it.
In this case it is arguable that your right to quiet enjoyment of the property, or indeed the right to have a habitable property, has been affected by the presence of mould for several months.
It really is a question of degree as to what impact this mould has had though. If it was present in large amounts throughout the whole property then there is more of a case for breach on the part of the Landlord. If it is a case of one or two small areas then this is a different matter.
It is disappointing that you had to complain on three occasions before any action was taken. However, action has now been taken and the property is, presumably, in the condition that it was when you moved in. This suggests that the Landlord has taken appropriate steps, albeit belatedly. In the absence of any lasting damage to your health I do not see that there is any significant claim for injury under s.4 of the Defective Premises Act 1972. Any such action would be of very low value and would require medical evidence on your part which, in itself, represents a significant expense.
It may, however, be worth stressing that your health was affected, albeit in a transitory way, when dealing with the negotiations to which I refer in the conclusion. This may strengthen your negotiating position and may make the Landlord more inclined to accept your point of view.
If anything you may have a claim for a rent reduction for the period whilst the mould was present but this would be very modest and not at a level where it would warrant Court proceedings. You should also beware that the Landlord may argue that the mould had been caused or exacerbated by lack of ventilation in which case it might be that they suggest some fault on your part. This is not uncommon in cases of this nature and serves to place some risk on the Tenant in the event that they purse such a claim.
In conclusion, it may be worth pressing the complaint further and asking for some repayment of or reduction in rent for that period when the property was affected by mould. I would not, however, suggest taking any formal action. I do not believe, from what you tell me, that you have valid grounds for seeking to abandon the Tenancy given the steps that the Landlord has taken to rectify the problems.
I am sorry that this is probably not the reply you were seeking but I do hope that it helps in some way. If you are content with the reply please rate my answer so that I can close off your question.