I would echo Andy's concerns, but would say the following, you need to speak to a specialist as soon as you can.
It is to be assumed that your DMP was set up on what you could afford 15 months ago, and you seem to have kept up with it with little problem, but you have also indicated that your circumstances have improved a little, am I correct in that?
As IVA's are based on affordability first, rather than level of debt, you need to work out, with the aid of a professional preferably, how much your disposable income actually is at present. Then there are various other factors to be considered. Are there any joint debts? Are there any debts in the 50k that are your wifes, or does she have other debts separate to you?
It is likely that if you go down the IVA route then you will be required to release equity at the end of the 4th year, if that cannot be done, for example if house values continue to fall, then it may be that you will have to extend to 6 years.
If your income IS subject to fluctuation, then maybe your DMP, to be followed by a release of equity in a few years to fund full and final settlement of the debts at a significant discount may be a better option. There is so much that needs to be looked at before a recommendation can be made, details which you may not want to share on a public forum.
As I said before, you need to consult a specialist as soon as possible, you may wish to visit
www.iva.com and have a look at the reviews there, I would certainly speak to 2 or 3 different providers before making you next move.
Regards.