part pay off my mortgage with a lump sum

Can anyone tell me if it is worth paying a lump sum into a mortgage half way through or is it better to wait until I have enough of a lump sum to pay off the whole mortgage. Do I save anything on interest at this stage or is it only when the mortgage is paid off in full earlier than the 20 years?

For example
If my remaining mortgage is 200k and I pay 1200 per month,
I then pay a lump sum of 100k and my remaining mortgage is now 100k
do I still pay 1200 per month?

If I do then presumably it would be better to put the 100k into a savings account like Rabobank at 3% pa and wait until I have the full repayment amount to pay it off in full.

Hope this makes sense.
Many thanks,
jemol

Accelerated lump sum or regular repayments of a mortgage will always save you (often significant) amounts on interest charges that you avoid. If you make a lump sum capital repayment and leave the monthly repayments as they were then you save money and reduce the effective term of the mortgage (thus also saving on mortgage protection life assurance premium payments when the mortgage ends early). If you leave the term as it was then your monthly repayments will go down and you will still save (albeit less this way) but the term will stay the same. See Karl Jeacle's mortgage calculator for information on how much etc. If you do decide to make a lump sum capital repayment then make sure that you put your request in writing to your lender so that there is no confusion about your intentions (e.g. so that they don't lodge the capital repayment as a credit on your account rather than using it do immediately reduce the capital outstanding).