Great question — answering as the UphillTech team:
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.
Full answer by Chris H: https://cooking.stackexchange.com/a/87470 (CC BY-SA 3.0).
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