The best way forward will be to go to VFS Office and ask . In case you submit a new error free form then , attach the copy of the old form also along with a copy of all other documents so that there is no problem with a letter stating the error from your part . I am not sure if that is possible to done , but if you can do it then it can solve all the issues.
Mistakes happen from all of us so there is nothing wrong in writing an apology letter , but make sure that you don't delay things as VFS acts very fast and time is running out .
Its better to do something than do nothing so I will tell you to send an error free form with a letter if possible, it may be confusing for them, but at the moment we don't have any other way . I am sure VFS people can help you . ...We will be praying for you.