The “Holiday Baking” special issue of Cook’s Illustrated magazine has a recipe for the pudding and and orange-mace hard sauce. They give detailed instructions for making it, including suggestions for faking a pudding bowl. Their recipe suggests you age the pudding for at least a week, and up to a couple of months before serving.
You may order suet online here…
You should try to make it using suet, I have learned over the years that people in the last few decades have been cheated out of the flavor and textures you get from using non processed natural fats. Butter, Lard and Suet are wonderful ingredients and should be used, especially for those once a year dishes that make a holiday special.
Here is a recipe for English plum pudding from James Beard…
http://www.epicurious.com/recipes/food/views/Superb-English-Plum-Pudding-20010?id=20010
Upon review I see you are preparing the dish for vegetarians, in which case I offer my advice for others. I’m afraid I don’t really have any good suggestions for a vegetarian friendly suet alternative. 
Just a clarification about one of the ingredients in the above recipe. As has been discussed before on here, “currants” in this case does not refer to black or red currants of the Ribes family. It actually means a type of dried grape similar to sultanas or raisins. The name “currant” is a corruption of Corinth (grape).
You should ask a butcher about suet. We use it in pasties, and they sell it fresh for birds too (besides the stuff they sell in blocks in the bird seed aisle) I live in Michigan, and it’s readily available here.