Remortgaging to raise funds - wait until fixed term ends?

Hi there

My fixed term Santander mortgage still has 18 months to run and I would like to add to the amount borrowed in order to do some work to the house. I am assuming this will be the cheapest way to raise the money in the short term?

However, I am assuming that if I attempt to remortgage now, I will incur early redemption charges, as I am not at the end of the fixed term.

Can anyone please confirm if I am correct and perhaps talk me through the process of remortgaging to raise funds. Am I likely to be able to do this now/in eighteen months?

Thanks.