Hi Steve and welcome to the forum
You definately did the right thing in increasing your payments, but it is a shame that you have not been afforded any support by your IP throughout the process. I am afraid, however, that your additional payments will not reduce the amount you will eventually have to raise by way of the re-mortgage or third party payment, unless the terms of your own IVA specifically allow for this.
It is unlikely that you will be allowed to pay savings monies into an ISA, unless they are monies out of your allowable expenditure. I cannot see your IP agreeing to increased pension or mortgage payments either, unless the mortgage payments are enforced upon you say by an interest rate rise.
IVA payments will have not effect on your child maintenance payments either.
Hope this helps.
Regards, Melanie Giles, Insolvency Practitioner for over 20 years.
To have me propose an IVA for you, please visit:
http://www.melaniegiles.com/ivaEnquiry.asp
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http://www.iva.com/iva_companies/IVA_Advice_Bureau.asp