Thank you Bronte/Sunny for your replies.
The bank in question is AIB and there is a formula they use for deciding what penalty (if any) will be charged in the event that some or all of the mortgage is repaid early. In there case it is:
early breakage cost = A * U * D% where
A is the amount repaid early,
U is the unexpired period (the period remaining to the end of the original fixed interest period) and
D is the difference in the interest rate (the difference between the fixed interest rate applicable at the start of the fixed interest rate period and the fixed interest rate applicable as at the date of the early repayment, for the unexpired fixed interest rate period)
In my case (were I to repay in October of this year) and pay off EUR50,000
A = 50000
U = 3.5
D = 3.86-4.25 (this is negative)
as D is negative in my case then "no early breakage cost will arise".
AIB have since confirmed this in writing.
My main concern was whether or not I would effectively lose my 3.86% fixed rate were I to repay some of the mortgage early. I have just contacted AIB and they have confirmed that I will remain on the same fixed rate and term, even if I were to repay some of the mortgage early, which is great news for me
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