I can only repeat Foggy's advice / thoughts, as expressed here:
tax-rebate-to-the-iva-t91775
Payplan have this to say: "
If I am due a tax refund, will all of this have to go towards my IVA?
Money owed from last 4 years
Tax that has been overpaid in the past 4 tax years can be recovered from HMRC. If you are in an Individual Voluntary Arrangement and have overpaid on tax, any money HMRC owes you from the last 4 years will be classed as an asset of agreement. This means your creditors are legally entitled to these funds which will go towards paying back your debts.
Money owed from the current tax year
If we find you are paying too much tax in the current tax year, HMRC will send you the money or pay it into your wages or salary and you may keep it. However, previous years’ tax refunds will go to your creditors and towards paying off your debts."
I am not qualified to give advice and can only state my opinions, based on my IVA experiences.