If there was no mention of a tracker on the other half, then you have no right to get it now.
Under the circumstances you should accept the offer. You have only got 6 years to rectify a mistake, so as they seem to have taken you off the tracker in 2008, you could miss out if you don't accept it.
Accept it.
Make sure that the refund is paid to you in cash, and not by way of reducing your mortgage.
Ask to see how the refund was calculated and get someone to check it for you.