Tenant eviction

Rent Arrears

Hi
This is a continuation/development of a previous enquiry.
I issued a Sec 21 repossession notice on Feb 1st. The tenant begged me to give him another chance and started to pay off the arrears, initially in reasonable sized chunks so I agreed to hold off with the proceedings. He has now fallen behind again and owes rent for most of May (he paid £100) and all of June. I wrote to him on 29th May and advised him that if the arrears were not paid up to date by the end of May, the Sec 21 notice previously issued would be revived (can I do this?) and that proceedings would be commenced against him for recovery etc. I have had no response to the letter or to repeated phone calls and messages. When I last had contact with him he was working away from the area. I checked the flat when I delivered the letter on 29th May and found evidence of occupation by people other than the tenant. The place is a mess and people have been smoking in the property (forbidden in the TA), possibly drug-taking. The front garden is a jungle which houses an old dumped settee. I now want him out as quickly and cheaply as possible. What should I do next? Since I have had no responses, can I assume he has gone away and change the locks? Should I deal with this myself or would I be better to let a third party handle it (perhaps the RLA). Speed is of the essence.
Thanks
Gordon Eaton