stop/start not working

Has anyone got a fix for Citroen C3 Feel 69 reg stop/start not working.
Actually I don't think it has ever worked or at least I have never seen the warning light on the dash light up.
I was having problems with the battery discharging over a period of a week and would have to boost start the car with a booster, contacted Citroen UK about the battery to see if it was covered under warranty but they informed me that the battery is only covered under warranty for 2 years and because the car has not been used regularly due to the pandemic causing lack of use.
My local garage tested the battery for me and said that it was faulty and only 51% charged and would not charge any more, I replaced the battery and my local garage reset and reconfigured the battery to the ECU etc but it could not get the stop/ start to work as it should.
My question is, is it possible or is there a relay that operates the stop/ start that could be causing it not to work because every now and then when I pull up and fully dip the clutch the stop/start warning light flashes briefly, but the car does not stop and start when I release the clutch.