Income support and compensation payment

I have an unusual case where a client received income support over a period of 5 years ago following an accident. He purchased a property within months of the accident as he was so far advanced in the process.

As he took out the addional amount of mortgage whilst in receipt of IS he is restricted to mortgage interest at the level of his lower mortgage prior to moving property.

He is now in receipt of Pension Credits.

He subsequently received compensation this year and the Income support payments for this period were duly offset and recovered.

His area of contention is that as he repaid the income support he was technically not entitled to Income Support for this period (in effect wiping the slate clean) and therefore should not be limited to a restricted level of mortgage interest. 

any thoughts or guidance?

Thanks