When to start rent recovery process (MCOL) and best recovery service

Rent Arrears

My joint tenants have not been paying rent since February just before the Corona-virus situation.
I have served s8 and s21 notices.
They may well not move out at the end of the AST tenancy (October).
The online service is a fixed amount claim, however the rent arrears are constantly increasing.
Should I wait until they move out or do something now?
I also want to pursue this, not for the money but for the principle that far too many scammers are getting away with rent theft.
As they both work a CCJ threat may persuade them to go.
Any suggestions for the best rent recovery service?
Thank you.