Hi Racoons
Welcome to the forum.
You might want to have a look at the following FAQ which explains where everything is;
www.benefitsandwork.co.uk/guides-for-claimants/faq/forum.
I have frequently requested extensions and never received written confirmation of the request.
What I do, I send the completed form off by recorded delivery, two days later I check with Royal Mail that it has been delivered and signed for, I then contact DWP to see that they have received the form and it has been added to their system, on a few occasions I have had to give the operator the recorded delivery number.
Once you know it is in the system you can relax, next stage will be the assessment.
Gary