British Naturalization Rejected - Help?

[TigerCub]

Hi all,

I am posting on behalf of a family member, and looking for advice.
The case is as follows - They are a EU citizen and have applied for British Citizenship, as they have been living on and off in the country for the past 30 years; as well as being married to a British Citizen.

For PR status they have been settled in the UK since 2006 - so PR status cam into effect as of 2011.
In November of last year they applied for the Naturalization after having received the PR card from the home office in the same year.

They had to request documents back to travel internationally for work in Jan, and received an itemized receipt from the Home office detailing the documents sent back, stating that the home office may need to review these documents at a later date but that they should keep hold of them until they are requested back from the home office.

Now a month later they have received a rejection based on lacking documentation - especially proof of permanent residency. In the letter received back it clearly states that a Permanent Residency document was returned to them from the home office.

My Question is how we proceed from here - do we need to fill out the NR from to review the case - as we believe that we submitted all documents requested in a lawful manner. Can our local MP help with this case, or is it best to seek legal advice?