Are you both in debt or just yourself? If you have equity of £42k in the property, and an ability to pay contributions of £850 for five years, this would result in a total asset pot of £93,000 against debts of £82,000. So I suspect this is where the advice that an IVA would not be suitable is coming from - albeit that does rather assume that you will sell your property.
An IVA would be based upon affordable monthly payments of £850 per month, together with the release of equity based upon 85% loan to value borrowings during the final year of the arrangement. It might work, but I have insufficient information available here to really comment further. If you have found an IP prepared to put IVA proposals forward, and this is at no cost to you, why not give it a go - as you have nothing to lose and if it is rejected you can then offer a DMP.
Regards, Melanie Giles, Insolvency Practitioner