[ubuntu] Dual boot Ubuntu on Mid 2010 iMac, no brightness control

Hello Community,

as my imac 2010 is become slow, laggy, limited updates available etc., I decided to have a go and dual boot Ubuntu. I am a linux user so was expecting some issues.
It is now mostly running ok, I am writing this post with the ubuntu boot, the most troublesome issue I am having is screen brightness control. I have spent a long time now ploughing through the various forums, trying different things. I am a little wiser however am no closer to a solution.

The sliders for screen brightness do not work at all.

Specs:

iMac 2010 with 12Gb Ram, intel core i3
Graphics card is an ATI Radeon HD4670
Ubuntu 20.04
Bootloader reEFInd

If someone here would be willing to guide me in the direction of a solution it would be greatly appreciated.

Thanks in advance.
Martin