Mine is a bit rough and ready and will vary according to the ingredients you choose, but it does work. It's more muesli bar than flapjack proper, tbh.
PUt 1/2 a pack of butter and 2-3 tbsp golden syrup or honey in a pan and melt together over a low heat, stirring occasionally. I don't let it bubble, but if you do the end result will be a bit more toffee-ish. When it's all mixed and melted take off the heat and stir in approx. 3 American cup measures of oats, either porridge oats or jumbo oats, or any combination thereof. Remember that jumbo oats are less absorbant, so you may need to tweak the quantities. Mix it all in well so there aren't any little pockets of dry oats.
Then (optional, but nice) you can add in mix-ins, any combination of the following: chopped nuts, raisins, dried fruit, chocolate chips, fresh fruit (either mashed, pureed or grated), seeds, whatever. This is a really good way of using up fruit that is slightly squishy or otherwise past its best. Choose mixtures that go well together, eg. apple raisin and cinnamon, pineapple and coconut, plain choc chips and orange, carrot and ginger etc etc. Mix it all in really thoroughly. The mixture should be stiff but movable and sticky all through; if it's too runny add more oats or even some flour, if too dry add a bit of an appropriate liquid (fruit juice, rum, or just water).
Turn the mixture into a shallow tin that you have buttered very thoroughly, and squish it down hard with the back of a tablespoon or a spatula. Bake on a medium heat (Gas 5 or 6) for about 25-30 minutes. Leave to cool really thoroughly (possibly overnight in the fridge) before cutting into bars and storing in an airtight tin.
This is pretty idiot-proof, and you can substitute ingredients as you see fit. It's hard to really mess up - pretty robust in the face of shameless ingredient substitution, and still edible (just about) even if accidentally left in the oven for an hour (not that I'd do that, oh no). 