Unable to clear purgeable diskspace (APFS)

I have a 4TB macbook, which until 2 nights ago was pretty much full. I attempted to back up my old iphone so that I could copy my data to me new iphone and received a message telling me I didn't have enough diskspace.

I have cleared well over 1TB of data off the disk but the iphone backup app still reports lack of diskspace.

It looks like I have 1.9TB purgeable now (this has increased from about 400GB to this figure as I've deleted stuff). Emptying the bin doesn't do anything.

My documents folder now apparently has 33TB of data in it...this would be funny if I hadn't wasted so many hours trying to resolve this????

Time machine is running so snapshots may not be allowing the disk space to be marked as available, however there are only snapshots from the last day:

I have to say that this user experience is completely ruining the excitement of getting a new phone...

I've done lots of reading and I'd appreciate any pointers as to what might be the problem...I'd like to reclaim my own diskspace!

Thanks,

Ipernicus