Remortgaging advice

Hi, this is my first post so hopefully I am in the right place.

I was wondering if I could ask some advice on remortgaging please.

I am currently in a parent child mortgage with my mum, but could now apply for a joint mortgage with my fianc!, however I am confused by the debt to income ratio. Is it just credit cards and unsecured loans I have to calculate or is it all expenses eg payments into my children's trust funds. Also are 1 month late payments going to affect the application? We have the odd one on our credit files.

Thank you