You will need to convince an insolvency practitioner that your disposable income is actually £350, as the firms that you have already approached seem to be suggesting that you can afford more. Could you post the details of your expenditure so that we can assist further.
The 25p in the £ benchmark is not there to work towards, but as a bare minimum requirement that a lot of creditors operate under.
With regard to pay rises, in my practice your income and expenditure is reviewed annually and rises are taken into account in conjunction with increased expenditure, and if your disposable income is higher then you will be asked to pay more - but of course the general spirit of an IVA is wanting to pay your creditors as much as possible.
With regard to getting married or living with a partner, your partner's income is taken into consideration, but only to ensure that you are both paying your fair share of shared expenditure based on the same ratio as your earnings.
Regards, Melanie Giles, Insolvency Practitioner for over 20 years.
To have me propose an IVA for you, please visit:
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http://www.iva.com/iva_companies/IVA_Advice_Bureau.asp