You will pay far more interest on your mortgage over a 20 year period than you would over a 14 year period.
To illustrate this i have just quoted some basic figures from my own website:
Total to pay on a £100,000 mortgage over 25 years at 4.75% throughout = £171,191
Total to pay on a £100,000 mortgage over 30 years at 4.75% throughout = £187,949
Total to pay on a £100,000 mortgage over 35 years at 4.75% throughout = £205,476
Total to pay on a £100,000 mortgage over 40 years at 4.75% throughout = £226,251
While the figures don't match your own you can clearly see the difference in taking a longer term on a mortgage and what will be repaid. The difference in figures between the highest and lowest repayment figure is purely interest paid to the lender (profit)
Why would you want to reduce your monthly payment and then make an over payment? If you are comfortable with you current payment then stay with the same term, if you extend your term you are likely to find a comfort zone with a lower payment and overpayments will fall by the way side and you will end up paying considerably more for your mortgage.
Go and speak to a mortgage broker and have this discussion, He/she will be better able to illustrate this to you in full and offer you ideas and solutions that can move you in what ever direction you choose.
If you want to save youself money you will decide NOT to extend your mortgage