Hi Paul
I do SAP calcs - http://www.sap-calculations.co.uk - (thermal heat losses for new builds, extensions and conversions) - as well as the other job - and because of your type of issue I no longer send out the calc with the invoice !! - the invoice goes out 1st and when its paid, then I hand over the documents.
I've used this company to recover debts (very cheap) http://www.thomashiggins.com/ but not on the scale you're talking about I suspect.
Unfortunately, this is the way of the construction industry, its build and pay for whats built (in stages)
Hope you get it sorted