Commercial cleaner owed money from estate agent

I am a sole trader and do cleaning work for an estate agent which includes end of tenancy cleans and a regular once a month clean to a building.
I am owed money for an end of Tenancy clean from June and I am owed July and August money for the regular building clean.
I have been nice and done numerous emails and spoken to them and I just get the usual dribble from their mouths.
Obviously don’t want to cause a problem because of the regular monthly clean (for which I have the only set of keys for the building) but not sure how to proceed. Do I now send an email stating I am going to charge interest?
I have decided I am not going to do September monthly clean until all payments are up to date.
Everyone else I do work for pays within 30 days and I have no problems.
Advice on how you would deal with situation would be much appreciated

Thanks for reply James,

I know she is a drain on my business, I emailed her 2 days ago saying it was unacceptable but have had no reply. I want to be firmer but do not want her not to call me when work becomes available. She has no problem with my work i always make sure it’s acceptable before I invoice her.
I need to be firmer but not sure how to put it to her? I want to add compensation but not sure how much, the first invoice for June is for £185 the 2 monthly cleans are for £200 each?

Update:- I emailed her on Thursday evening to her office email address, didn’t expect a reply and got one within an hour! I told her she had 7days to pay or charges would be added and then past onto 3rd party to collect, told her it’s nothing personal just business.
Her reply was she is in Greece and will sort ASAP!
She has access to internet because she replied, I know she has internet banking because I have been with her when she has paid me immediately!
Let’s see what happens