non payment on shorthold tenancy

I have tenants on a short hold tenancy (6 months).they are 4 months into the tenancy and have only paid a miniscule amount (1/2 of one months rent).They have given excuses for non payment , to which i fell for.They now state they want a 'Notice to Quit' order for next week.I still have the bond but it does not cover the rent.Can anyone give advice as to what actions I should take.thanks