put all your ingredients except the oil in blender and start it up to make a good base..
add the oil slowly and you will notice the mayo start to thicken.. here is where it can all go wrong
thyere will come a point, usually before you have added all the oil, so the taste is still to acidic where the mixture will need a bit of water.... you will notice there is no more movement in the mayo..... if you keep adding oil without thinning it first it will split... it means your recipe quantities are not quite right
also i find eng mustard works better than the powder
i usually make mine to taste without using a recipe.... you make nice variations on the mayo once you get to there
red peppers, garlic, chilli, spring onions... whatever fancy really
Hope that helps
Chris