Assuming no overpayment charges, this only makes sense if the interest rate on your savings is greater than that of your mortgage.
OP has already said he has 3% overpayment charge within his fixed term, that is why I mentioned wait for the fixed term to end (April 2018 as confirmed by OP) and save up as much in the meantime, when fixed term ends dump all the saved money off the capital with no penalty.
Also if overpayment is going to be regular I would look at a product that doesn't penalise for this as I have done.
FYI OP I am also a contractor but when I rang Santander as I was staying with a product they offered I didn't need to go through the whole 'affordability' process (which was lucky as I only had a 3 month contract at the time).