I think you're right - just had a look on my XC60 and there is just one negative cable from the battery to a grounding post below. There's like a braided metal strap earthing the engine - not sure where this is on the 1.6D but I'd check the gearbox to engine bolts.
Interestingly I found this on ebay - it might be the one you referred to ? Looks like someone has had the same problem at some point!
https://www.ebay.co.uk/itm/274777449...kAAOSwPEBgQke5