This is where different mindsets come into play. Some people hate having debt and would do anything possible to reduce that debt. Id wager if OP has 10k free to put against mortgage capital that they dont have other debt to worry about, and that they are probably savers too. People with this mindset get money, look at how they can improve the future with it, and if there is anything left over afterwards then great, if not - well the future has been improved.
The other mindset is the one where someone is managing your finances in a particular way, and if any extra shows up the see it as 'free' money to be enjoyed in the current moment, not as a way of improving the future, and not as a way of clearing debt. These are people who dont mind so much about having debt.
I dont know who is right or wrong btw - Im just musing about different peoples attitudes to money.
Anyway OP, I asked my bank this very question recently and was told that paying money off the capital does not in fact bring down either the repayments or the term agreed, unless you re-negotiate the mortgage (this is relevant for people who do not want to change their terms, if they have a good deal that they may not get today), but what it does do is allow you to pay off the mortgage in less time than the term allowed for.
You should also check that there are no penalties for paying a lump sum off the capital with your bank (depending on your mortgage type).