Basically, if you are able to save this kind of money, you I&E review was done incorrectly and you have over declared your spending. The principle behind an IVA is that you pay back, not just the estimated payment sum, but, as your income increases / expenditure decreases, so does your disposable income / IVA payment. In a nutshell your allowances should be based on your actual spending. Take these away from your income and what is left is your IVA payment.
If your Help to Save account matures within the term of the IVA that, too, will be payable into the IVA as a windfall if over £500 ( I am referring to the bonus element only).
If CF are happy with your I&E and you can make these savings by living frugally it would be best to tuck them away somewhere and not draw undue attention to them. A F&F offer from those savings could be poking a sleeping bear with a sharp stick !
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014