trevor
No, the DWP do not confirm an extension requested by the claimant, however, they can phone back to check that it has been noted on the computer.
If they can afford it then I would say the form back using a Tracked post so that they have proof of delivery, failing that they should ask for a Certificate of Posting from their local Post Office.
Gordon
Nothing on this board constitutes legal advice - always consult a professional about specific problems