Mortgage Overpayment Help

I have a mortgage with Nationwide on a 10 year fixed with 31 months remaining set at 4.89%.

It will cost me £1500 to ditch the fixed rated.

I am aloud to pay £500 overpayment per month which I do.
I also put £1500 into premium bonds per month too because of the £500 overpayment limit.

I have £100,000 left to pay.

I have done the ditch the fix calulator and it advises me to change. however my question is.....

How much would I save if I ditch the fix to 2.59% where I can pay the full £2000 in overpayment straight of the mortgage compared to what I am doing at the moment?

Thanks for your help in advance!!!