I think my confusion came from this scenario.

Tlog backup at 9:30 and 10:30 and a full backup at 10:00

The full backup at 10:00 contains transactions since the last full backup

I thought that the 10:30 tlog backup just contained transactions since 10:00 (the full backup) but as I have now been told it will included transactions going back prior to the full backup, back to the most recent tlog backup at 09:30, so technically, transactions made between 09:30 and 10:00 will be in the 10:00 full backup and the 10:30 tlog backup.