Shared ownership staircasing question

Cheryl2022

Hi,
does anybody know how the shared ownership Mortgages work please?

So my situation is I currently have a 25% share in my house I owe 23k mortgage on this and my share is worth 55k so I have in theory 32k equity.
i am wanting to purchase the full house which has been valued at 220k in total which the housing association have said I can but how does the mortgage side work? 

Do I have to apply for a brand new mortgage for the whole amount of 220k and my shared ownership mortgage will get paid off or do I apply for the mortgage for the remaining shares worth 165k? The reason I ask is I won't be able to go with my current mortgage provider as I had a couple of credit issues in the past so they won't lend me the full amount but other mortgage companies will? I'm so confused how it will work please help if you can? Thanks