Decide Whether to Overpay Your Mortgage

You have surplus each month and two sensible homes for it: knock down the mortgage or invest. This prompt treats your mortgage rate as the guaranteed return from overpaying, weighs that against likely investment returns, and factors in risk, flexibility and how you actually feel about debt. You leave with a reasoned call, not a gut one.

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You have surplus monthly income of {MONTHLY_SURPLUS} and are deciding whether to overpay your mortgage or invest the money instead.
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<task>
**Step 1: Calculate the guaranteed return from overpaying**
Your mortgage interest rate is the guaranteed return you get from overpaying. State this clearly and compare it to cash savings rates.

**Step 2: Assess the expected return from investing**
Compare the historical long-run return from a diversified equity index fund with your mortgage rate. Account for the variability of investment returns vs the certainty of the mortgage saving.

**Step 3: Factor in tax and allowances**
Have you maximised your ISA and pension allowances? If not, that changes the calculus.

**Step 4: Make a recommendation**
Overpay / Invest / Split, with a brief explanation of the key driver.
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- A comparison: guaranteed return vs expected investment return
- A tax and allowances check
- A recommendation with the key reason
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