I can sympathise with your frustration having gone through this myself this week. My Form was to be in by 8th April and was posted back on Sunday 3rd April so that would have given plenty of time to be received.
I was shocked to receive this week FOUR envelopes with my P45, Tax Notice, Notice of payment and finally a letter to say that my Benefit had stopped on 8th April as I had not sent my form back
I got on the phone to JC+18th April to be told that my Reconsideration for the lateness had been accepted and my form was received Sunday 17th April - What a cheek - never asked for a Reconsideration as the form was in time.
Although the Lady dealing with me on the phone was apologetic the system allows people to be penalised for their misgiving.
What will happen if i need to make a Reconsideration - Will they say you have already had one??? We live in hopes - But thanks to B+W for the guidance.