I split my money as 'part in savings, the remainder in a certificate'. What happens if the total changes?

When you split with the wording 'a set amount here and the rest there', the part you named with a figure stays fixed, and the part called 'the rest' grows or shrinks with the total. So if you find extra money or spend some, it is the certificate that changes, not the savings account, unless you say otherwise. This is not just arithmetic: the new certificate amount can enter another tier or pass the minimum of another variant of the product, so the return is computed again, and any offer or figure made on the old amount should not be relied on. Note too that a savings account's return is computed differently (balance basis, tiers and fees), so it is not added to the certificate's return in one figure. If the amount changes before you lock it, ask the bank for a new offer on the new amount, and make sure it is written down which amount goes where. Check the certificate's key-facts statement for the minimum and its multiples and the amount tiers.