nicholson
The DWP should have acknowledged your appeal within two weeks, although this has extended to 4-5 weeks due to most offices being overloaded, so If you have heard nothing then you should contact your local office to find out what is going on.
With regard to your records, the DWP maintain at least two sets of records for you. One is stored and maintained on the DWP computer system, you appear to have received a copy of this. The second, is a physical file containing the documents (eg. ESA50, medical report, etc.) related to this claim, there may be other files containing docs from previous claims, although these tend to be destroyed after a period of time.
It may well be that your physical file (this may be referred to as an ESA55), has been moved between offices, and that is why they are having problems locating it. I would keep pushing for a copy of it as you are entitled to. If the call centre are unresponsive, you can escalate the issue to the Customer Manager, although this usually requires a call back.
With regard the appeal, the DWP have to submit a file to the Tribunal Service containing all of the information used to make the Decision, this process is independent of any SAR you have submitted, and you should still receive this information if the DWP do not revise the Decision in your favour.
Gordon