Referring back to the email about chasing the payment.
A lot of companies run their budgets from 1st Jan to 31st Dec, and as such, depending on how the finance director wants to play things they either cram a lot of outgoings through at the end of the year to use up an allocated budget, or try and hold off until the next year so they don't go over budget for the current year. This is all very common, and could possibly explain the wait for the milestone payments. Will January be our month?