Some discussion of this issue here: https://www.flyertalk.com/forum/brit...payment-2.html

Summary: They may give you grace, and there have been examples of that, but note the warning that you should not rely on it happening without action on your part. At a minimum, call and discuss.

I can't quite make sense of your dates though. I thought the norm was for the final payment to be due 35 days (5 weeks) before travel, which doesn't tie up with being due in a few days but with travel in May - I don't know if the due dates can vary so maybe in some cases the final payment is due earlier. Also, is it just a typo, but 4th March is a Wednesday.