Bad tenants

Possession - Section 8 and Section 21

I have, or rather 'had', 4 tenants sharing a house on a STTA which is due to end on 4/6/13. On 18/5/13 with the rent exactly 2wks in arrears they informed me that they will be moving out on 15/6/13 and that there would be no rent forthcoming as I had their 6wk deposit! I have also been told that I may do the Inventory Checkout the day they move out. I have now discovered that 2 of the original tenants returned to Australia some months ago, to be replaced by I know not whom - it seems to me that a fair deal of sub-letting has been happening. Though rather late in the day now, my Solicitor favours having CCJ's served on them through 'Money Claim Online' for rent and repairs? (These could be extensive judging by a visit I made with a Letting Agent). I am thinking of serving them with a Section 8 Notice on 5/6/13 on Grounds 8(rent arrears), 10(rent due for the extra 2wks to 15/6/13), 12(broken obligations), 13(deterioration of house) and 15(deterioration of furniture). Is there anything to be gained from either course of action, or really anything I can do at all? I can't help thinking what is the point of pages and pages of Tenancy Agreement, 6wks or even 2months 'Protected' Deposit, Detailed Inventory check-in and check-out, etc, etc when it all amounts to nothing in the case of tenants who stop paying the rent 1 or 2 months before leaving - for Australia??!!