If they do replace your battery, check that you have actually got a new one and not a “spare” from a van that has been sitting in their yard for months. They (Preston van centre) near as damnit admitted that this probably happened to me when my original Moll battery was changed under warranty after 2 years, and I am now having problems with the replacement Moll again a year later, though it tested OK a week ago when I took it in. It’s currently reading 12.24v after driving 71 miles on Saturday but just 8 on Sunday. Thankfully it’s a TSI, so usually starts OK.
It does sound as if your van has further issues though, but they might be caused by the battery?