Is the property let only or fully managed by a letting agent?
If the latter, they need to be involved at this point. If it is a let only and the landlord is holding the security deposit, you may have a probelm getting this back from him. Working in the lettings industry, I advise your friend to look at his/her contract and there should be a claus relating to situations that make the proprerty unhabitable and are not acceptable etc - along those lines anyway. I would then as the previous post suggests write to L, which may lead to him doing the job. Perhaps then speak to Citizens Advice or obtain some legal advice.x