All measurements and instructions are for guideline use only- I don't cook by scales.

Base-

300g of digestive biscuits
100-200g, probably.. melted unsalted butter

Bash the biscuits to breadcrumb consistency, add the butter in bits until it just starts to hold together. I tend to add too much butter here and leave the finished product a bit greasy.
Leave to set for an hour or so. The mix should look fairly dry, but cling if you squish it against the sides of the bowl. I find this balance the hardest part to achieve.

Take your tin and smear the crumb mix around- squish it against the sides of the tin and just make it look thick enough. Leave it until it sets, should be an hour or so.

Filling-

325ml - 3/4 ish pint of Double Cream.
150g dark chocolate
2 egg yolks
Tablespoon and a half of plain flour
A good sized knob of butter, about 50g or so.

Melt the chocolate into the cream to make something like a good hot chocolate mix, you could add spirits here- a cointreau one decorated with orange zest has gone down well in earlier and slightly more bitter incarnations.

You know it's right When you're willing to serve it in a mug. When it gets to that point, add the yolks, flour and butter. From here, it starts to act like a roux and will get badly lumpy if you don't pay attention. Mix it until it starts to thicken and then pour it into the case. If done right, it will start to develop lumps that vanish when you stop stirring. Get somewhere near that and all mistakes disppear when set.

Put it all back in the fridge, and leave it to set. It's ready in an hour or so.

If I was doing this as on-the-day with a roast, I'd get the base in the fridge, put the roast in on the half hour sizzle/seal and then make the filling. If it's on a low heat, by the time that's done and poured, it should be about time to turn the joint down. The cake should be close to setting by the time food is eaten, and I can spend at least an hour chatting and doing nothing more than a quick baste. But then, I only cook for friends.