Preheat the oven to 190 degrees C.
Line cookie sheets with non stick parchment paper.
Beat together the butter, tahini and the sugars for a good 5 minutes until you don't see any sugar crystals. Add the eggs, one at a time and beat between each addition to mix well. Add the vanilla and beat again.
Measure the flour and bicarbonate of soda into a bowl and, using a sieve, sift this mixture into the bowl with the butter. This is to get all the little balls of bicarbonate well blended in.
Add the salt, oat bran, and flax seed and stir well. It is a stiff dough but if it's really too stiff, add a splash of milk.
Add the chocolate chips and mix them in well.
Using two little spoons, drop blobs of dough on the prepared cookie sheets - I usually do 12 per sheet because I don't like it when they stick together.
Pop them in the oven and bake them for 10 minutes or so. They should not look anaemic although I appreciate that people like them gooey.
Remove them to cool on a cooking rack. When they are completely cool, pop them in a tin to keep. They will last a couple of weeks in a tin. You can also freeze them.