"It can be done but it is entirely at the descretion of the life insurer"
If there is a conversion option then subject to the policy T&C the insured has the option to extend both the sum assured and the term. Insurer discretion doesn't come into it.
"Most shouldn't have a problem provided the risk hasn't materially changed i.e. you haven't been diagonosed with something in the last two years."
Again a health issue would not affect cover if a conversion option can be exercised.
"That said, the older you are, the less likely they are to agree to extending the cover"
Again under a conversion option the insurer cannot refuse to cover you as as they would a healthy person of 41 years of age.
If there is no conversion option then subject to underwriting (a) propose a new policy for a 25 year term and (1) keep existing policy as a form of life assurance because you bought it when you were younger and rates cheaper or (2) look into the possible financial consequences of terminating the existing policy once you have the new policy or (b) reduce the term of your loan to 23 years, keep existing policy and propose a new 23 year policy but only for the difference you need (€100K). Note if your new loan is a repayment type loan then decreasing term assurance (mortgage protection ) is cheaper. In a mortgage context, level cover is usually used to cover an interest only loan.