Should I ditch my fix for Offset?

I have a fix rate of 3.69% until Jan 2015, currently have £147,544 left on mortgage over 23yrs 6mths. I have come into a large sum of money (80k) and I am wondering whether to ditch my fix and go onto a offset as savings interest rates at the moment are not very good. I will have to pay a £4300 ERC if I ditch my fix but Yorkshire BS are doing a good 2yr fixed offset rate at the moment of 2.49%

I've tried doing the maths and I think after 2 years I will be in credit by more than I would have earnt on interest?

Opinions would be grately appreciated as very confusing!!:huh: