Bms negative battery lead
 

Hi,i think I may have a faulty bms lead on my S60 2016 as it sometimes says start stop available almost immediately after starting a journey,then without using start stop,it will go to unavailable and I can travel 30 miles without it coming back at all.
Both batteries have been replaced and I have reset bms previously.
My question is,I have a lead coming from Ebay,does anyone know if the bolt on the inner wing to the chassis has a captive welded nut or does the inner wing lining need removing just to replace the lead ?

Thanks