Hi @Johan
While I believe the answer to your “is this very serious” query, is probably better to be answered by your accountant, I wanted to reply and help find that missing £13 so everything matches.
The first thing to do, is to reconcile your bank account on QuickFile against bank statements: Reconciling your bank
Start at a wide period (e.g. month or quarter), and once you identify a mis-match, start narrowing it down to weeks and days.
Hope this helps!