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Apologies for the belated reply! You can do either, but I found it easier to heat the top and drag it. I heated only as much of the skewer as fit over my hot plate. It takes a lot to heat it up enough to work, so I couldn't be bothered waiting for the entire skewer to heat up.

Let me know if I miss anything as I have a habit of getting carried away and missing parts!

1. Definitely. I had a couple of leftover slices I'd left for others to claim that were left in the fridge for 2-3 days. They went unclaimed so I ate them and found them much easier to get the spoon through the pastry. Custard and cheesecake will keep for a while. The thing that deteriorates the quickest is the pastry as it eventually goes all soggy in the fridge. But on the slice, it makes it easier to deal with. I'd refrigerated mine overnight (approx 8 hours) and it required all my patience to cut. The sugar lattice especially is difficult to cut through. One way you can navigate this is by using scissors to cut through the top layer of sugar/pastry, then use a hot knife to slice through the rest.

2. I'd add the gelatine when adding the yolks. You could theoretically do it at any stage and the gelatine will stay liquidy when stirred over heat. An even simpler way would be to use gelatine leaves and stir that in once the milk is hot.

3. You could use vanilla essence/extract. As you said, the flavour won't be the same, but it will still work as a flavour enhancer as well as giving it a hint of vanilla. I rarely measure when using vanilla essence - I usually just use 1/2 a capful or so.

4. I didn't refrigerate between. It's usually a bad idea to put hot stuff in your fridge (it can facilitate bacteria growth throughout your fridge by reducing the temp inside). I baked the puff, had that ready on the bench while making the custard, poured the custard on top and then covered the tin with glad (plastic) wrap while preparing the cheesecake layer. You can let the custard cool a little and then put it in the fridge. It will begin to set as it cools to room temp. If you're adding gelatine, don't make them in advance. If the mixture sets before you have a chance to pour it you will go through hell trying to get smooth layers.

5. The skewer thing is a technique I've heard of before but never seen. It took me a lot of trial and error to get it (mainly it is very easy to underestimate how hot the skewer needs to be for this to work). You could practise beforehand but I'd suggest trying on a tray. If the skewer gets really hot it can ignite a flame when touched to the sugar and I'm not 100% certain that parchment paper could withstand it.

Yes! Icing sugar, not caster! Thank you for pointing that out.

That Bird's stuff looks like the right thing. If it helps, I use Foster Clark's brand, which lists its ingredients as: "Cornflour (maize wheaten), sugar, salt, colours (102, 110), flavours". The exact content of the powder shouldn't matter too much as I add so many extra things to the custard the custard powder ends up acting just as a thickening agent with a little extra flavour.

I made the lattice after refrigerating overnight so that it's easy to get out of the pan. I think did the lattice, took photos, then popped it back in the fridge for later. The lattice did remain after refrigerating, but it's not as clear and distinct. It's best to do it soon before serving, but if you do it a few hours before it should last.

I would definitely make this again. Everyone loved it and it is pretty quick and easy to make in comparison to most desserts I do. I've had several requests to do so and my boyfriend has this on his list of favourite desserts ever (and keeps dropping hints for more). I usually wait a while before repeating stuff as I prefer to experiment with new stuff rather than revisiting something I've just made, but it's likely to make several more appearances in the future.