If you have a bank statement (paper or online), enter a date range you wish to check, and the amounts on those dates:
Press ‘Go’, and it will tell you if there’s a difference between those dates:
Repeat the process using different date ranges unless you can narrow it to a small date range - that should help you get to the issue quicker than ticking transactions.
Edit
The other option would be to download a CSV export. There’s a useful guide here that explains both methods:
As always, if you need further help, please don’t hesitate to ask 