bed bugs

Letting Agents

I was recently contacted by my letting agency to say a tenant had reported bed bugs in one room of my 5 bed HMO and did I want to send in an exterminator? I said Of Course, they told me the tenant had previously reported them a few weeks earlier when I had NOT been informed by the letting agent. SO the first time it was reported by the tenant the letting agent had told her to 'hot wash' all linen and but a bug bomb in room, which I think was a reckless decision considering how devastating bed bugs can be. Had they informed me of this first report I woudl have sent an exterminator in straight away. The exterminator has been to the property twice now but this week two more rooms now have bed bugs in. Im so furious this has been dealt with so badly and now have angry tenants.
so A) how do I deal with bed bugs in a 5 room HMO and b) who is financially responsible. I feel it is the letting agents fault for not informing me fully when problem first arose and then being stupid enough not to send exterminator in immediately.