I think thats a reasonable letter and offer on your part. They get away without having to fork out the 8% interest and you get what you want.
At the end of day Lucy, only you can decide whats right for you and your circumstances. Its all well and good chasing them for every single penny but in some cases it may be just as accaptable to take what is offered and walk away with peace of mind knowing that your curent situation has been eased and that you have gained a moral victory.
Whatever you decide will be the right choice for you..........and at the end of the day thats what matters.