I'm a bit surprised that this ran for 12 months before eviction. Once they were 2 months overdue you can use the s8 route plus an s21 to evict them. The s21 is mandatory for the judge to order an eviction so long as you have filled out the form correctly. What went wrong?
Also, you dont say why the debt collection agencies wont take this on. Do you have details of where your ex tenants are now? Are they working? Do you know if they can repay the debt that youre chasing? If they have no income, on benefits etc etc, then you may find its just not worth chasing them as you will never get paid.
Try posting your question on a landlord advice site such as this
http://www.landlordzone.co.uk/forums/forumdisp lay.php?f=3
where you should find others who have been down the same route as you.
Sorry I cant be of more help.