DLA to PIP application

I filled in my PIP form and posted it on 29 December. I registered it to prove I had sent it on that date because it was due in on 13 January. On the form I told them I would be on holiday 7 - 21 January, however, on 5 January I received a phone call inviting me for a medical on these dates so I was told they would write to me with a date for the medical. On 6 January, PIP wrote to me saying they hadn't received my PIP form. I can prove my form was sent on 29 December, that it was tracked, that it was signed for. And if it hadn't been received why was a company inviting me in for a medical? I was medically retired by Atos when it was decided I needed constant care and my employers couldn't be expected to provide it. Has anyone else had a run around such as this?

Thank you, Gordon, for your reply. I have a medical/meeting on 23 January with the health professionals working for the Independent Assessment Services, so we'll see how it goes. Atos decided to medically retire me from work because they felt I needed more care than employers could be expected to provide during the working day, and I have sent a wealth of information backing my medical retirement, however, they still want to see me. To say I am a bag of nerves is an understatement. Thank you again for your reply.