I've commited the change roundup-trunk-r4483-xapian-1.2-take2.patch    
to Revision 4484 with the minor change:

-        # XXX: Xapian databases don't actually implement transactions 
yet
+
+        # XXX: Xapian now supports transactions,
+        #  but there is a call to save_index() missing.

Olly, to close the issue, could you test if this works on Xapian 1.2
and could you provide a patch to make the transactions work or 
alternatively open a new issue for it?

Thanks in advance,
Bernhard