Getting our bad tenants out

Rent Arrears

We have had tenants for 18 months, slow payers but now it is more than 2 months in arrears. We sent a letter asking to discuss this & they now no longer will answer the phone or answer the door, although we know they are in and we probably shouldn't have come round. We have always had good relations till now.
We have properly registered the deposit, but did not send the prescribed info pack.
Do we have to pay a fine before we can take legal proceedings??
We cant seem to get any further as they wont talk to us. Help!