...top up automatically with €15 each month. So why then does my account show a balance in the region of €23?
Only speculating but you might not have been buying bundles all the way through since March. Top up gives calls + 15GB. Credit older than 3 months is gone so there's possibly 30 euro accumulated but with texts excluded from the top up offer, the 7 euro difference could be for sent texts. What does the usage history show, if there's a charge shown for any text then it was outside bundle which indicates the bundle wasn't active.
Also, calls to 1890, 1850, 0818 etc are charged for as they aren't included in the bundle.
Also check whether or not the bundle is set to auto renew. Auto top up might not necessarily mean bundle auto renew.
Anyway if you've checked all that already then don't know why there's that amount of credit built up.