PIP - Appeal Length of Award?

Hi everyone, Is it possible to appeal the length of an award?
I have sent in a review form a month ago for my wife who gets PIP and I am still waiting for a response. The original award was for 3 years and the review form arrived about 12 months early.
We are applying for a mortgage as we want to move house but I have been told we need a letter from the DWP showing that the award is either “indefinite” or “for life”.
Do they make such awards? If the award is only for another 3 years can we appeal?
My wife’s condition is a genetic, incurable, progressive one and she gets (at the moment) enhanced rate for both care and mobility; her condition will never improve and there is no treatment.
Thanks for listening.

Thank you Gordon, good idea. There are other grounds too, see below. On a practical level can I withdraw the MR? I had felt DWP had been fair to me initially, would the Case Manager I spoke to have been that person? But a four year award, bit unusual?
I am being assessed and investigated for more mental and physical health conditions......maybe insist on help from DWP visitor to do the next review?

I got an ATOS 0/0 report but DWP awarded me 15/4 on my evidence which HP didn't bother to read. Nor did she write down anything I said at f2f. But I'm very stressed out atm, worried I might lose what I have although I thought I should have had higher scores.

I struggle with forms, over attention to detail
I also find it hard a to monitor or explain the minute details of my various Mental & Physical health conditions. My decision was made on a MR as DWP cancelled my claim. I read this was contrary to their own guidelines, when dealing with ppl with certain health conditions.

I had low rate care & mob DLA awards past 10 yrs so they had info on this and taking ages to get forms in each time. I got mob for needing help to go to unfamiliar places. I struggle due to a cognitive impairment, the ensuing chaos, brings on the psychological distress.

Also, it suggested to me, and I'd welcome your opinion - was to email JC+ Area Manager, including copies of 3 letters, ask him to intervene? Await response, cancel MR if positive. Feel bit bad complaining re that CM re decision. But I told her I was having difficulties, shouting without realising or intending to. Not very sensitive, she said she was going to hang up on me twice. I asked when the MR was due and she shouted I just told you! I said I was probably still concentrating on what you said b4 that. I have profoundly inattentive but combined ADHD.

Also should I pop in a SAR request? for all copies of files held by DWP, about me?

Moderators fear I may have given too much info in my post awaiting approval.
As my situation is quite specific. My indefinite 10 year award was reduced by 6 years to 4?
If you agree, I am happy to edit and repost?
Or feel free to do so yourselves.
I am articulate but my traits of OCD & perfectionism, GAD, ADHD cause me real problems with forms
Thanks again