Rent payment less than agreed

Rent Arrears

Hi Eveyone,
My tenants have rented from me for over a year without problems, however for the last few months they have been late paying and this month they have eventually paid £50 less than the agreed rent.
They have become very difficult to contact - not answering phones, emails etc however we finally contacted them and they told us that they have applied for social housing and have given us verbal notice to leave - but they not given us a date so I can't advertise for new tenants.
I'm really not sure how to handle this situation. I am not prepared to let them mess us around and pay what they feel like but I do realise I could be much worse off if they didn't pay anything at all!
I would appreciate any views/advice.
cheers
Faz