Flapjacks.
Ok, I think I've about got this down enough to share now.
Total cooking time, under an hour.
Ingredients
250g Porridge oats
125g Salted butter
75g Golden syrup
75g Soft brown sugar
The oats need to be half 'premium' whole grain oats, and half 'crap' chopped / rolled oats. The bitty oats help the whole thing to bind together, the finished product tends to drop to bits otherwise. Rather than buying two separate sets of oats, I use Quaker Oats, then stick 125g in a ziploc bag and take a rolling pin to them until they're crushed but not quite flour. It's a bit more work but less boxes in the cupboard.
Method
Set the oven preheating at 170'C, and line a baking tin with greaseproof paper (I'm using a 6"x8" dish).
Melt the butter, syrup and sugar together over a medium heat until it's all liquid.
Remove from the heat, and tip in both sets of oats. Stir it all together well, until it's balling in the pan and there's no sign of dry oats.
Spoon the mix into the tin, and press it down with the back of a spoon. Whack it into the preheated oven for ~25 minutes, until it's starting to look toasted. Don't panic if it seems 'loose' at this point, it gains cohesion as it sets, and if you try to get it firm straight from the oven you'll end up with shortbread.
Cut it into flapjack-sized pieces. I get eight slices out of my batch. Take care when cutting, it's delicate at this point.
Leave it to cool overnight, take riding in the morning.