I had a VERY large American IT company hold on on me for 9 months.
As you say, no interest paid on that.
I had a contract, but then you get into the costs, both legal and professional, of going to court.
In the end they did the right thing, but only because they needed me and I was sitting at home refusing to do any more work until they settled up.
At first I wasn't too concerned, I've been contracting my entire professional career and so am used to and prepared for long gaps between payments. Got a little out of hand though.
In the end I still work for them however an agency sits between them and me now, and I get paid when my timesheet is signed and the Agency can deal with the lengthy payment terms....
BillA, lets turn this around. If you haven't been paid for the big radiator review, who owns it?
Next, if you proffer a written contract and begin work, even if it is unsigned, it can be considered in force as the work was done under the offer of it. Little grey there, but I remember a contractor who refused to turn up to work as his new contract had different wording to the old one.