Hi Baz and welcome to the forum
Your NatWest debts will be treated as three separate creditors, and the overdraft will need to be included. If you do not owe any money to HSBC you can keep the account.
Your IVA will affect your partner in that your finances will be separated out, and she will need to pay a share of the bills in the same proportion as her earnings - and in certain circumstances, your creditors may also wish to have her disposable income offered as well as your own.
To find out whether you would qualify for an IVA, and also be made aware of other options available to you, it is best to seek direct advice from an insolvency practitioner which is generally offered free of charge.
Regards, Melanie Giles, Insolvency Practitioner for over 20 years.
For further details contact me at
http://www.melaniegiles.com and view my IVA blog at:
http://melaniegiles.blogs.iva.co.uk