If you were not aware of the windfall at the time, but were technically entitled to it and have discovered this entitlement retrospectively, then you do need to inform your IP.
An entitlement to an asset is relevant at the time you become entitled to it, and this is not necessarily the same as you became aware of the entitlement. At the time of your grandmother's death, did you have cause to suspect that you may be entitled to an inheritance? If so, honesty is the best policy, so how much are you actually entitled to inherit, how much did you owe to creditors in the IVA, and what final dividend was returned?
Regards, Melanie Giles, Insolvency Practitioner