Making a money claim for unpaid rent

Tenancy Types and Management

I have tenants who to date owe me £879.20 in unpaid rent. On top of this I have informed them that I will be charging interest (as per the Tenancy Agreement)and also charging expenses of £10 for every letter sent reminding them of their rent arrears for the duration of the tenancy (a total of 8 to date).

My question is at what point can I make a claim to recoup the amount owing? I have been granted a possession order (with 28 days for the tenants to move out) on 23rd August so am anticipating a possession date of around 21st-ish of September. I was planning to make the claim online through the HMCS money claim website but do I have to wait until they actually move out or can I start it now?

Also, is there any way I can make sure I have a forwarding adress if they do move out?

I am 100% sure they will not pay the amount owing as they have not paid any rent personally to me since they moved in (I have received some housing benefit payments however, all of which I can provide evidence of if needed).

Thanks in advance
Caryn Davies