Backservice.app using lots of energy - no idea what it is (after having followed guides to remove it) - is it a virus??

You should be able to get more information/details by selecting the process in Activity Monitor and right-clicking it (or one of the icons on the toolbar).

Uninstall the anti-virus software by following the developer's instructions. Anti-virus apps, cleaning/optimizer apps, and third party security software are not needed on a Mac. These types of apps usually cause more problems than they solve, plus they impact system performance. macOS already has great built-in security especially when users also practice safe computing habits as outlined in this article written by a respected forum contributor:

Effective defenses against malware and other threats - Apple Community

Now download and use MalwareBytes to see if it will identify & remove "Backservice" since a quick search reveals this is a browser hijacker. MalwareBytes is the exception to the above rule about not using anti-virus software (every rule seems to have an exception). The reason is MalwareBytes is less intrusive than any other anti-virus app and it is more familiar with macOS malware & adware than other solutions. After running MalwareBytes to remove the malware/adware, there is no reason to keep it...either select the "uninstall" option (on the app's "Help" menu), or at least turn off MalwareByte's real-time scanning protection.

If you still have a problem, then run EtreCheck and post the report here using the "Additional Text" icon which looks like a piece of paper. This report will provide information which will help us analyze your Mac and show what software is configured to run at boot & login.

Shiny@UCL wrote:

I was able to right-click on it (which I couldn't do via activity monitor as it was greyed out) and find where it was -> Application Support -> Clever files. From there I was able to delete the lot.

I thought "Clever Files" sounded familiar, it seems it is Disk Drill. I do recall another thread on these forums which mentioned some unwanted app like "Backservice" being installed. Seems like Disk Drill itself or the server the app was downloaded from is adding in other stuff to the main download. Keep this in mind when downloading third party software...make sure to only download direct from the developer or a developer approved mirror. However, even some legitimate download site servers will end up bundling in other unwanted junk many times without alerting the user.