I am using the following article to come to my conclusions.
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In the article it says that you can increase your mortgage up to the value of the property when the property became a let property.
It doesn't say anything about only being able to claim up to the value of the mortgage at the time the property became a let property.
Here's the example in full
Case Study B – Benefiting from letting out your own home and increasing house prices
John and Louise buy a house in 1987 for £50,000. They live in it for 14 years and then decide to move to a bigger house. Instead of selling the existing house they decide to let it out.
Over the 10 years the house has been fully paid for and therefore there is no outstanding mortgage.
The value of the house in 2001 is £190,000. The new house that they have seen is £300,000.
In order to provide the deposit for the new house, they re-mortgage their existing house on a buy-to-let mortgage for £152,000. This amount is used to fund the deposit on the new residence, which means that they only need to borrow £148,000.
The entire interest that is charged on the buy-to-let mortgage can be offset against the rental income. This is because it is below the market value of the property at the time of letting.
Two years later the original property is worth £250,000. In order to reduce the mortgage on their private residence, they are able to release an additional £38,000 of equity on the buy-to-let mortgage. This means that they have mortgaged to the amount of £190,000 on this property.
Again, because they have not gone over the market value of the property at the time of letting they are able to offset the entire interest charges against the rental income.
At the same time they have also successfully managed to reduce the interest on their private residence by an additional £38,000!
Once again, if the original property is remortgaged above £190,000 and the money is not used for the purpose of the lettings business then the interest charged cannot be offset against the rental income.
End of Example
Therefore using this example I would expect to be able to increase my mortgage to 60K (the value of the property when it was first let) and get mortgage interest relief on the whole amount