2 Mortgages - how do I calculate my options?

Ginger_Winner

I currently have the following two mortgages:

1. C&G - 'Let to buy' (capital repayment for the property that I used to live in but now rent out)

Property Value: £179,000
Loan left: £153,000
Term: 11yrs, 11months
Rate: SVR, currently 2.5%

2. Abbey - Capital Repayment (on the house I live in now)

Property Value: £385,000
Loan outstanding: £217,000
Term: 21 years
Rate: SVR, currently 4.24%

Both of these were previously on trackers which have just expired so I'm currently reviewing my options.

To add further complication to this, it would be beneficial to me from a tax perspective to maximise the interest on the let to buy mortgage as this is classed as a cost in terms of my tax return.

What I’m considering doing is increasing the term of my let to buy mortgage and using the annual saving that this will give me and overpaying my 2nd mortgage by that amount.

I don’t know how this adds up and I’m not really sure how to make the calculations, any advice would be gratefully received as well as an explanation as to how I can make these calculations myself (I’m guessing there must be some on-line tools to do this out there….?)