The usual rules in cases like this are that tenant requests landlord to carry out repairs. If landlord does not comply within a reasonable time frame, then tenant repeats request giving landlord a specific time by which the repairs are to be completed. Tenant also submits quotations from contractors to complete the repairs, and if landlord again fails to effect these repairs, tenant does so using the lowest quotation and deducts this sum from the rent paid to the landlord.
Any information given in this post is based on my personal experience as a landlord, what I have learned from this and other boards and elsewhere. It is not to be relied on. Definitive advice is only available from a Solicitor or other appropriately qualified person.