Lost ESA1 claim form - please advise!

I completed my mum's ESA1 claim form on her behalf because she is too exhausted from chemotherapy to do it herself, and she can't concentrate for very long/ doesn't currently have a landline so calling them to make a telephone claim would be difficult.

We posted it on 22nd April and Royal Mail tracking shows it was received on the 24th - we have a signature, initials and date/time stamp. The address we used was, as late as May, confirmed by the DWP premium rate number call centre to be correct and obviously someone did sign for it.

ESA Worthing, Post Handling Site B, Wolverhampton WV99 2FS

However, we have since been blamed for posting it to the wrong address and I have since been given *four* other addresses by different call centre staff. These are as follows:

Worthing Benefits Centre, Freepost DWP ESA 36
Brighton Jobcentre, 30-35 Edward Street, Brighton BN2 0LN
Brighton Jobcentre, Post Handling Site B, Wolverhampton, WV99 1PT
Jobcentre Plus, Brighton Jobcentre, James Watt Way, Crawley, RH77 1AB

Please note, my mum lives in Brighton and that these address all appear to relate to either the local jobcentre, or the nearby town of Worthing which appears to be the geographical area by which new claims are sorted.

We need to post another claim form. I have it all ready to go - along with proof that they lost the previous claim form in the hope that they will backdate it and accept our photocopies, since original documents were also lost in the same envelope - but each time I enquire, I get given a completely new address. I don't know what to do.

My poor mum has been without social security for 8 weeks despite having cancer, and what with no record of her having any claim because they lost it, we can't even get an emergency payment. I'm tempted just to show up at the jobcentre and demand they scan it in, but assume I'll be turned away. I could try to get an appointment there, but I've already spent *A LOT* on the DWP premium rate number and can't afford to keep calling them only to get nowhere...

Any advice would be very much appreciated. We don't trust them not to lose it again and don't have a clue where's best to send it.