Usually you cannot change any terms of a fixed rate mortgage without paying a penalty so this means no overpaying, no paying back lump sums and no paying back of the mortgage early. Fixed means a fixed interest rate and a fixed monthly repayment.
How strictly this is applied differs from lender to lender, it looks like EBS have become more rigid in relation to overpaying. They are free to change their policy so if the conditions of the new loan offer do not allow overpaying then you will not be able to overpay.
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