I am having similar issue at the mo and you need to take a couple of steps:
1. You need to inform the LL and agency of the problem in writing i.e. a posted letter. Email, text or phone will not suffice. We sent a registered letter to ensure it got there!
2. You need to allow them sufficient time to respond to the letter, in this case 2 days is probably enough. We called about our problem 4 weeks ago and only after the formal letter on Monday is it being sorted!
3. If the LL/Agency fails to respond you have two choices, get it repaired yourself and invoice for the amount, you cannot take it out of the rent as you need to ensure that you are fulfilling you side of the tenancy. The other choice is to issue notice to the LL and move out after 28 days.
4. If you do repair it yourself and there are any problems getting your money from the LL, your next port of call is the PRTB.
This came from threshold who I spoke to last week. Contrary to what I thought, I cannot force my LL to do repairs! He isn't liable to provide alternative accommodation even though our property is inhabitable! Its a joke, so if you want to stay in the property (as we do), then follow the process above. Make sure all communication is recorded in writing, you will need this in case there is a dispute.