Hi Desperate Dan
Given your debt is £40k, I would always be considering an IVA quite simply because by staying with a debt management plan, it is going to take you for ever to get out of debt. Even if you are currently paying £500 a month, it will still take you a minimum of 80 months (nearly 7 years) and that is if all interest and charges are frozen for good which is not likely.
However, there are a few points to consider. The first is you can only do an IVA if the equity in your house is less than your debt. How much equity do you have in your property currently and is it owned jointly with a partner or spouse?
In terms of your IVA payments, I agree that you would need between £250-£300 a month. If in this context you can afford to change your mortgage to repayment then you can certainly do this but you would need to do it before the IVA started.
In terms of the re-mortgage element of your IVA, yes you would have to agree to do this (normally at the 5th year of the arrangement). At that time, the equity in your property is reviewed and if you are able to release any, you will have to do so. Your insolvency practitioner will introduce a mortgage broker who will work with you to do this.
If no equity can be released, you may have to extend your payments for another 12 months to compensate the creditors.
Given your situation, I would not want to wait another 6-12 months before starting an IVA. You can not put money aside before you start as you would probably have to give this to your creditors. As such, the key is to make sure your expenditure budget is correctly calculated so that you have enough each month to live on and for eventualities once the IVA starts. I have done this for people I work with many many times and it does work well.