Address not coming up on Royal Mail PAF

geocatwest

I've been living in my flat since November 2018. 
Long story short - the address I'm living at is not registered on Royal Mail Postcode Address Finder but is on the electoral roll and council tax band valuation. It is not on land registry as a registered address.
I found out the landlord originally had 14 flats but converted it into 28 flats, so each flat has two units. The council argue that this is illegal and the landlord is currently going through an appeal. I can't wait for this appeal as I'm unsure whether it'll be successful. Even if successful, the landlord needs to go through building control and also register via street naming and numbering.
Applying for mortgage - this will obviously caused an issue with credit reference agencies. Before this month I did have everything registered at my parents' address but only recently changed - would it be too late to change this back?
Should I use my parents' address?
Or should I do the obvious thing and try and leave as soon as possible and try and rent a new place? If I rent at a new address, do they check your former addresses as well?