Mortgage Interest Tax Credit

10 years ago, my wife and I took out a joint second mortgage against our jointly owned main home to purchase a jointly owned second home. We used the second home ourselves (for work reasons), but with retirement have now rented out the second property. Given that the additional mortgage on our main home was used solely to fund the purchase of the second property, can the interest from that second loan now be applied for a tax credit against rental income from the second home?

Hi ASTJW,
The emphasis is the purpose of the loan, not how the loan is secured.  
The capital element is not deductible.
To the extent the loan is used for the rental business, then the loan interest and any loan arrangement fees are allowed as a deduction.
Note that from tax year 2017/18, the tax relief that landlords of residential properties get for finance costs is gradually being restricted to the basic rate of income tax. You will need to calculate your tax relief for finance costs for residential properties differently.
The amount of tax you pay will stay the same unless your total income (without a deduction for the finance costs) exceeds the higher or additional rate threshold.
For guidance, please see:- Tax relief for residential landlords: how it's worked out  
Thank you.