Invoicing for work not completed

In May we were sent an invoice for the first progress payment for "Piers"
We were surprised by the efficiency - especially as the piers were only started later that day (and completed one week later).  Which is no big deal, but seeing as this was the first progress payment (and that payment terms were only 7 days which is a bit cheeky) I though it best to inform one three designer homes that we would make payments in accordance with the contract, not earlier, and would make the payment within 7 days of the piers being completed.

7 days later there was a reminder that our invoice was due, I again advised that  that we would be making any payments in accordance with the contract, not earlier, and in an attempt to avoid further problems also requested not to receive future progress invoices prior to completion of the work as specified in the contract.

Good we should be OK from now on then....

Later in May we received an invoice for the next progress payment.  Here's part of what the contract says about when this payment is due:

bearers, joists and floor sheeting laid.

Below is a picture of the 'house' taken 3 days after the invoice for this work was sent.
The bearers, joists and floor sheeting were completed at the end of August.  More than 2.5 months after the original invoice was issued.



This was initially just annoying as it just felt rude and greedy.

It became concerning though when, in discussion about the early invoices one three designer homes became quite pushy and then almost desperate about getting us to pay early.  I was puzzled why one three designer homes continued to push for the payment when the contact was so black white about when it was due and at this point I started to become concerned whether there were cash flow problems at one three designer homes.

These concerns escalated when I stumbled on the story of the collapse of Beechwood homes.  Fortunately no such problems occurred but annoyingly this wasn't the last invoice we received before the relevant work was completed.