Hi @Nigel2410 and @Vintage97 and welcom to the forum.
I firstly would do a software repair using Xperia Companion, which is encouraged after a major software update such us Oreo update. Then, I would wait about a week for the battery to be re-calibrated.
Another thing you could do is a manual re-calibration following this procedure:
1. With the phone turned on, charge it to 100%. Then, disconnect it.
2. Turn off the phone and connect it again. If it is not 100% charged, the battery may not be calibrated.
3. Turn on the phone and discharge the battery until the phone turns off by itself
4. Charge it again to 100%. Then the battery should be calibrated again.
