Mortgage overpayment Vs savings

Mshrts

I know the advice is to keep any extra cash in savings if interest rate is better and overpay at end of fix
However is this still true where mortgage value is high over a 30 year term?

I want to overpay £5k per year for next 5 years which is remainder of my fix.
If I overpay each year do I not save a greater value of interest over the full term Vs interest for just the next 5 years? 
Does such a calculator exist? 

777birdy

following this as wondering the same myself  

AliceBanned

I also wondered about this. I’ve been overpaying for 2.5 years and my interest rate went up 1.5% on my fix in March. I’m just carrying on with small overpayment as before until I have an answer to this! Not sure whether previous gains from op are lost when you stop op if that makes sense! Especially when the rate has gone up.