Not sure, but if you've got a signed agreement, proof that you've provided the work as agreed and proof that they've not paid, they surely don't have a very strong case?
Have to say that it's the money side of things I'm finding hard with my new business. Feels awkward asking for money up front, bit pushy afterwards 
I do hope you get it sorted easily - perhaps they've just forgotten they've not paid [hopeful emoticon]
By the way, no fab plans for this weekend. We are off to see my MIL and step-FIL - would normally be really looking forward to it. Now just a bit apprehensive, but after all your fab advice on here from you ladies, I'm planning on going as if it is a totally normal weekend with them within reason - I'll take my lead from her as to how much she can cope with the children climbing all over her and competing for attention obviously!!