Explain that you have submitted your query via the online site twice and received no auto-response, otherwise they will just ask you to use that.
Use the following format for your query to get the quickest response:
Mail Centre / Location Worked:
Total hours you worked (inc breaks):
Total Hours you have been paid:
Total Hours you are missing:
Additional Notes (if applicable):
In my experience it can take up to a couple of weeks to get sorted (as they email your manager and wait for response etc) but it always does get sorted in the end.
Also, if you are able to, you can ring or visit the “book room” or “duties office” at your site and ask them to confirm what hours were actually submitted.