The short answer
Begin several months before the fixed period ends. Ask your lender what rate and repayment will apply, then compare staying, refixing and switching. Include legal, valuation, cashback and break costs rather than comparing headline rates alone.
A fixed-rate expiry can change the monthly repayment automatically if you do nothing. The earlier you understand the options, the more time you have to gather documents and compare properly.
Your review checklist
Ask for the maturity options.Request the lender's available rates, terms and estimated repayments.
Check your current position.Note the balance, remaining term, property value, loan-to-value ratio and any overpayment rules.
Compare the total cost.Include legal and valuation fees, incentives, cashback conditions and how long you expect to keep the mortgage.
Prepare for a switch.A new lender will assess affordability and request documents, so allow time.
A broker may help compare lenders, but ask which lenders they cover and how they are paid. If you stay with the existing lender, you should still understand why that option suits your circumstances.
Sources and further reading
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