Ulster Bank are probably the most flexible when it comes to fixed rates.
You can overpay by 10% of the outstanding balance each year. Depending on your current balance that should give you some additional flexibility. It's as simple as setting up a direct debit for the extra amount.
Alternatively you could shorten the mortgage term to match your desired monthly repayment.
Both options would give you the desired results but be careful with the second option as you're permanently shortening your mortgage term. This might place unnecessary pressure on your finances in the future. However, the first option gives you the flexibility to cancel the overpayments at any time you want.