There is quite a lot of legislation relating to the mutual obligations of Landlords and Tenant, most of which clearly stipulates the types of repairing obligations which fall to the landlord and tenant respectively. Although it is not possible (or rather practical) to list ever repairing contingency which the landlord is responsible for in the legislation, the problem you described seems to fit well with exterior drainage which in all likelihood is your landlord's responsibility. The National Renters Alliance regularly assists clients with repair matters and it is quite common for us to find clauses inserted into the shorthold assured tenancy agreement which attempt to shift repair obligations to the tenant. Typically these clauses are null and void and do not stand up to scrutiny if the dispute goes legal.
If you would like us to look at your case in more detail please get in touch with us directly either by telephone or by email at firstname.lastname@example.org