There are some things you can do.
1) First make a record of all contacts with the agency - and their replies.
Like when you called - who you spoke to etc.
2) Contact your local council houseing - I think that now all landlords have to register with the local council
and the council have a dispute resolution service for this type of thing.
Do you know any details that may help you contact the landlord directly -.
How do you pay your rent - from which I guess the agency get a cut. If its by bank s. o. then stop the order.
IF by cash or cheque then do not pay - but e mail (keep a copy and a copy of any reply) and state that you have been unable to pay your rent as you are unhappy re the issues.
But in all this you have to be reasonable etc - You do not say what the issues are so I guess it must be serious.
This is about getting up and makeing a noise and getting heard.
Hope this helps.
http://www.shelter.org.uk/
This is the site for Shelter - thier site see under Repairs on the right hand links.
They give lots of good advice - for those in Private lets.
HOPE THIS HELPS IN SOMEWAY.
ALL THE BEST.