I have a Sony Z5 compact which I’m quite happy with but will be unable to get work stuff on it soon because the Android version is too old.

Has anyone managed to upgrade the version of Android on their phone outside of the manufacturer’s updates? Googling this comes up with a lot of guff and dodgy sites – surely someone here knows how? Or if.

Your best bet would be to flash a non stock rom such as Lineage to it, have a look on the XDA forum below for the best info.
https://forum.xda-developers.com/z5-compact/orig-development

I’ve used CyanogenMod a few times. Discontinued now and replaced with lineageOS. Always been good for me. As above XDAdev is the place to look

Android 7.1 is not that old in Android world!

I know, it’s fine for me but doesn’t meet work’s requirements.

Will check out XDA thanks

Yeah have a look on XDA. Depending on the phone it will either be reasonably straightforward, a complete ballache or not possible.

A rooted device with a non factory OS will almost certainly not please your work IT dept either I’m afraid.

Sounds to me like work needs to be providing you with a new phone which is fit for purpose.

Try a full factory reset first – worked wonders for a few months on my old phone.
Work needs to get you a new phone.

XDA. I just put Android 9 on my old Oneplus 2