management co allowing squatters

Letting Agents

my sister and I rent out a licensed HMO in Salisbury and in August last year we put the property in the hands of a letting agency, Whites, to manage. We had a good rapport with one of their staff who was super. He left in around autumn to go back to sales. the arrangement we had was that 2 of the residents were long term and we had vetted them and collected their rent so they wouldn't charge us for this and they would manage the other 3 rooms and collect their rent minus the 10% management fee. One of our original tenants rang and said there are 4 men living there now, and her. She's very reliable. So this means someone is occupying the spare room that Whites ought to have advertised. I rang the agents and they were awful. They did not know which room was which & said the 5th room is archived so they haven't advertised it and had no idea who was living where. My sis and I are furious, not only have we got a squatter, but we've been paying his share of the bills but we have missed out on rent for the past few months. They are now saying they want 10% from the 2 original tenants. They seemed entirely shocked that some other person was living there and whites had no idea and were shocked that someone was living there without permission, until I pointed out that an empty room and someone needing a home would soon be filled by one of the existing tenants friends, probably. I want whites to get this person out, but I don't know if I should be asking for more. Any advice?