Hi Francis,
First of all, what date had you written in your application form? Was it before 6 April or after it? If it's before that, they MIGHT ignore your mistake as you have just post it 4 days later.
Secondly, have they taken the money from your account or not yet? If yes, your application is under process and you should keep waiting for their call. If not, the best thing to do is getting in touch with them as soon as you can and ask them what you should do.
Depending on your application type, they are being processed within 6 months. So instead of waiting, try to call them and consult with them.
Good luck!