As noted on a previous thread about this from Jim Stafford..."one way to present your case to the Appeals Panel would be to present a "Before and After" computation i.e. show what the present day value of your pension fund today with what the present day value of the fund would have been if you had kept up your pension contributions."
To do that, I do need to engage an actuary to calculate it.
But I will add, I'm not really focussing or bothered about the return that may, or may not have been achieved if I had put the overcharged amount into a pension fund...it's more about missing out on the tax relief and my pension being under-funded for the period of over-charging. I was over-charged over 40k. Had I put that money into my pension, regardless of any gains on the fund - it would have increased the fund by about 70k (given the tax relief). I can't just put the refunded amount into my pension fund now, as I'm already putting in the maximum allowable. As a result, I feel I've missed out. There's no where else that over-charged amount would have gone except into a pension fund...I had no debt other than my mortgage and I had savings appropriate for my income /lifestyle.