How to store a Christmas pudding to make it drier?
A storage method that will dry out your pudding will essentially be staling it. Starches retrograde (lose moisture) fastest between 17F and 46F (-8c and 8c). Unfortunately, this not only causes the starches to release moisture, they also revert to their crystalline form which can make them unpalatable. If you still want to do this, the refrigerator is the perfect place to keep it in this temperature range, but it will still take some time to lose moisture. 

Alternately you could speed evaporative moisture loss by storing it somewhere warm and dry (a low oven or over a pilot light?) Again, this will probably make your pudding dry in an unappealing way as the outside will dry faster than the interior.