Apple Devices software has just bricked my iPhone12 Pro

That message is usually associated with antivirus, vpn or 3rd party firewall software interfering with the connection.

Disable or remove any you have installed. Note that disabling them may not be sufficient. You may have to completely remove them.

Make an appointment at the Genius Bar. they can restore iOS there.

You may want to try from a different network, and possibly using a different computer to eliminate a connectivity problem as the culprit.

wyldbore wrote: "It is on it's original battery but it is at 80% so not completely useless. It was on a USB>lightning cable anyway so would be powered off that I think."

wyldbore ~ 80% max capacity is not healthy and so could well be the reason for that problem. Apple suggests having the battery replaced at around 80% – so have the weak, aging battery replaced:

iPhone Battery Service

About genuine iPhone batteries

Repair Provider Verification

(Articles found by searching Apple's Knowledge Base.)

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What do I do now?

Have you contacted Apple Support?

Support

It's been a year or so, but when I used Apple Devices on a PC, I had to turn off the anti-virus in order for things to work correctly.

The iPhone will need to be erased and restored to factory default more than likely. Then just restore from your backup.

It needs to be erased and restored from a computer, not the device itself.

wyldbore ~ As your iPhone 12 Pro could be >5 years old, the problem could be due to a weak, aging battery. ...Does your iPhone 12 Pro still have its original battery?

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Does your battery show health as normal or something else?