Bent, sorry to hear of your predicament.
It all depends on if you're on speaking terms with anyone with any clout IMO.
I will probably get lambasted by a number on here but I am quite laid back about most payments-sooner or later almost everyone pays IMO.
I would call them up and say-in your friendliest voice, 'Hi, it's Benny the window cleaner, (don't use a company name-try for the more informal approach) I'm supposed to be cleaning your windows next week, bet you can hardly see out if them eh? (still keep it casual) its just that you're three payments overdue and I can't clean till they're all paid up. I'm due on Wednesday, can you leave a cheque for £600 please then that'll bring you upto date"
Then, Tuesday afternoon 3pm ish, call again saying "just confirming tomorrow's clean and that (whoever it is) has left my cheque"
If they say no. Small claims court.
If they say yes, and its there (don't clean till you've got it), great!
If they say yes, and its not there when you arrive you know they're pulling your plonker. Walk away, calmly.
Go home, file your claim.
Some people are just rubbish at paying.
There are (again IMO) very few actual malicious customers.