Cannot remove bootcamp from Mojave 10.14.5 fusion drive

I no longer need BOOTCAMP on my iMac Mojave 10.14.5 fusion drive so tried using Boot Camp Assistant to remove it, but get the message:

"Your disk could not be restored to a single partition.

An error occurred while restoring the disk to a single partition. Please run Disk Utility to check and fix the error."

I checked internal drives:

Fusion Drive, Container disk2 and Macintosh HD with Disk Utility First Aid and got no errors, can't run First Aid on BOOTCAMP.

Still get the error when trying to remove BOOTCAMP.

Container disk2 is disk0s2, disk1s2

Macintosh HD is disk2s1

BOOTCAMP is disk1s3

So I am stuck, how do I remove the BOOTCAMP partition and add it back to my Macintosh HD?

TIA,

Davo.

PS: It may have something to do with my recent reinstall of a clean Mojave on Macintosh HD (I didn't touch BOOTCAMP) and then restored all my stuff from Time Machine. BOOTCAMP boots fine and Windows 10 is OK, just don't need it any more.