Which mortgage to overpay?

Langerman

Hello all, I currently have two mortgages; residential £134k (property value £350k) 2.18% (4 years left on fix) with 23 years remaining - currently overpaying by £100 a month. My second mortgage is a consent to let £74k (property value £170k) 2.5% variable. I am simply trying to find out mathematically which is the better to overpay, monthly or as a lump sum. My instinct says the one with the highest interest but I can't help think the one with the biggest debt as surly it accrues more interest ??!!??. I have tried to find an online calculator but to no avail so I thought i would ask my first world problems on the forum. Your help is much appreciated.

MM10

i can see how this gets tricky. How many years are left on the second mortgage

getmore4less

are you letting and claiming relieve on the interest?

follow on rates are relevant

The size of loan is not relevant.
The length of term not relevant unless you can change them.

dark^knight

Personally, I would pay off my residential one first as it would mean no one can take away the roof over my head...but that's just me.

Langerman

Hi, there are 23 years left on my residential mortgage.