Nationwide mortgage rejected - reapply?

I am trying at the moment to buy out my partner of the house we bought together. I need to stick with Nationwide as I need to try and avoid paying the Early Repayment Charges if I went to a different lender. We were going to change the name on the mortgage and extend the term.

I am with a new partner now and we did a joint application. It was rejected for 2 reasons:

- my "funds run low" at the end of each month so they have concerns about affordability

- he has gone into his overdraft and there are missed payments on his credit score (he was out of work for 6 months due to mental health but has been back in work since January and has cleared all debt accrued)

Is it worth us reapplying for this mortgage in 6 months time? I can evidence more funds in my account at the end of the month (I'll just keep the money in my account that normally goes into savings each month). He can evidence that he hasn't gone into his overdraft but I'm not sure how long missed payments stay on his credit score for?

Any help appreciated - we have two children and I'd really like to keep the 3-bed house we have rather than going into a 2 bed flat!