Thousands of families have mortgages linked to Halifax's standard variable rate (SVR) - the most basic option, which stands at just 3.5%. But in an extraordinary clampdown, the lender won't let borrowers stay on this rate if they move home.

Instead, they're being offered expensive (to arrange and to service) fixed loans, and in some instances no longer have access to the best deals, after falling house prices have changed their loan-to-value ratios.

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