Hi there @Feargal_83402 I'd be happy to look into this for you, I'll just need a little more info so please let me know if you checked your top up history, was a top up of €5 or €20 completed? You'll be able to see this in your My3 account by clicking 'View History' and checking 'My Top Up Details'.
Also, can you confirm what plan you're on, If your plan is one where €15 is the activation amount €15 of the €20 would have been used automatically to activate your plan, leaving a balance of €5.