overpay or save for a bigger deposit?

72chapeltown

We have approx 110k left on a tracker mortgage at 0.89%, 25 years still left on it. We want to buy a bigger house in approx 2 years time. Should we overpay the mortgage each month, or put the money into a 3% cash ISA to then use to increase the deposit on the next house?

Logic would dictate that we put it in the ISA as it has the higher interest rate and is disposable, but we still have 25 years left on the mortgage and it's a much bigger sum. We would be looking to buy a house for about £250k and can save approx £400 per month.

Am I also right in thinking that car finance is ignored by mortgage advisers because the car is technically owned by the finance company until it is paid off in full? i.e. should we overpay this loan by £100 to pay it off earlier, or use the money to overpay the mortgage/put into the ISA?
Very confused about what is the best option. All advice gratefully received!