DSS nuisance

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New tenants moved into my property on 17/01 at an agreed rent of £440pcm. Being in receipt of benefits, they told me that they would receive full LHA of £450. However, this is not the case and they are only going to be getting £375 (confirmation now received from Council)and are in no position to pay the shortfall. The tenancy agreement clearly states that rent is to be paid in advance, yet I have not received any payment.
I have told them that I need to discuss the matter with them in person but they are refusing to do this saying that they want to deal with everything in writing instead. I want these people out of my property asap- can anyone please advise me as to how I can do this?!

Thanks