I know this a few years old now but just wanted to say thank you for the heads up on the earth cable.
I've owned my 2015 4LB tdi for a couple of months now and it's always been slow to turn over, starts no problem just slow turnover.
Was about to order a new battery as didn't want to get caught out during winter when I stumbled across this thread.
Just popped out and put a jump lead on the negative jump start tag and the other end onto the body of the Aircon pump and what do you know, spins much faster.
Additional earth strap ordered.
Thanks again 👍
Kevin.