Hindsight is a wonderful thing.
The picture I have from that book has the Abbot with four german soldiers, two of which look like they are highly ranked. The Abbot did survive by staying in one of the cellars. The person I'm trying to contact for my own information is the son of a man who commanded troops during the battle. I'm sure he would know where their command headquarters was located.
You can read all about it in Owen Chadwick's magnificent Britain and the Vatican during the Second World War