Don't panic!
You will definitely need a valuation and information on your mortgage, because otherwise your creditors won't be able to compare your offer with the predicted alternative, and they simply won't vote.
If your IP agrees that you won't be able to get an affordable remortgage, then you should ask them to make this point in your full and final offer as it will help your creditors to decide.
As for a reasonable offer, remember that your IVA will have been accepted on the basis of an expected dividend. If you can offer them a reasonable dividend and have a good reason for not offering the full amount, then it should be accepted. Only Payplan will be able to tell you this though, as they have all the information regarding the assets and costs of the arrangement.
Suggest an offer to them and ask them if the dividend returned to creditors looks reasonable. They should then advise you on the chances of it being accepted, and give you the opportunity to discuss matters with your boyfriend.
Best of luck.

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