Who is liable to pay rent shortfall

Tenancy Types and Management

Hi, would anyone know answer to this.
I let out an HMO property to 6 students on one tenancy ( RLA shorthold tenancy agreement) at £1950pcm, one student moved out during the first month of the tenancy due to illness, I was asked if I could help by tenant that moved out, so I re advertised her room seperately.

I managed to let out within 2 days, just before term started, however, new tenant moved out before after 2 weeks.

Original tenancy is still in place, but new tenant signed to say she was taking over the room from the girl that moved out to replace her.

With it being so quick, I did not recieve garuntor form back off new girl, nor has she paid her full months share of the rent, her share was £325pcm, she paid £300 plus her share of the deposit £400

My question is are the five remaining tenants liable to pay share of girl that vacated, in section 1a it states, We will let the property .... to you ( individually and together if there is more than one of you)
It is an eleven month tenancy starting from 01/09/2012