thank you. The position in relation to joint tenants is different as between the fixed period of the tenancy and after the initial fixed period. During the fixed period of the tenancy, all the joint tenants need to give notice together to end the tenancy so therefore, to exercise the break clause validly, all of you would need to give notice jointly. If one of you does not, then the break clause is not validly exercised.
By contrast, after the end of the fixed period, anyone of you can give notice on behalf of everybody whether they like it or not and that notice would end the tenancy for all of you with or without their consent.
From what you describe, you are presently still within the initial fixed term of the tenancy but have the option of a break clause however, as we have discussed above, you are reliant in this respect upon your joint tenants jointly exercising the break clause with you and from what you describe, they may not be willing to do so though of course you could potentially revisit this point with them.
If they refuse however, then you would need to look at replacing yourselves as tenants as I understand you are presently trying to do. It is understandable that the current condition of the property in particular, the need for works is putting off prospective tenants in this regard. You could ask the landlord to reduce the rent which I understand the landlord is open to doing to account for the works that are needed and the reduction in the enjoyment you will have from the property in this respect. The issues you describe are clearly unpleasant but in themselves, they will not be sufficient to justify a repudiation of the tenancy on your part-that is to say for you to terminate the tenancy unilaterally. For this, you would need to show that the property is largely uninhabitable.
Accordingly, if you are not able to secure the agreement of your joint tenants to exercise the break clause, your options are relatively limited in this respect to attempting to secure a replacement tenant on the basis of the condition of the property as it is in the assurance that works are to be carried out which I agree may not be straightforward and demanding a reduction in rent from the landlord to account for the condition of the property and the associated loss of amenity. It may be that if your joint tenants are unwilling to cooperate in respect of the break notice, you may have limited options in terms of securing a replacement tenant until such time as the work is completed. You could of course offer to pass on some of the rent reduction or indeed all of it to any prospective tenant as an inducement to take over the tenancy before any works are completed which can be an effective approach in encouraging interest