There have been strange developments recently at Tate Britain. First we had a near controversy-free Turner Prize, won by a painter, Richard Wright, whose work could actually be described as beautiful. Now, the Tate Christmas Tree 2009 has been put on show, “Weihnachtsbaum” by Tacita Dean (until December 23) -- and it turns out, of all unexpected things, to be Christmassy. Indeed, it is a delightful, almost magical sight.