Keeping uncooked Christmas pudding mix
In the end I tried it, or something similar.  I made a batch of mini puddings, but only 4 out of 8 individual pudding basins fitted in the slow cooker at a time.  So the second half of the batch went in the fridge for 7 hours while I cooked the first 4.  The second lot were cooked overnight, starting from fridge cold, but with the water topped up to as deep as I could go, straight from a boiling kettle.  The two batches are indistinguishable -- I've sampled both.

What didn't work so well were the few I made by microwaving in silicone muffin tins after I ran out of pudding basins before using all the mix.