I just finished my application and saw on the page this notice that may be of interest to those of you planning to apply tomorrow.
"There will be planned maintenance of the online visa service between 20:00 BST on 23rd May 2019 and 00:30 BST on 24th May 2019. You will not be able to access or submit your application during this time."
So please take note and plan ahead if this affects you. And best of luck to us all.