Ride with your stirrup ONE HOLE longer, and DO NOT pinch with your knees! You should be able to easily imagine someone could slip a piece of paper between your knees and the saddle when you are riding. Strength comes from gripping with your CALVES. If you fix this muscle memory and get your upper leg in a more appropriate position, fixing your toes will be easy; but the real problem is in your knees.
Also, pay attention to how you walk and stand when you aren't riding too. I used to walk a bit 'duck footed' and stand with my feet turned out. Paying attention to that and making and effort to almost walk 'pigeon-toed' eventually helped me break the habit of turning my feet out in the saddle and on the ground.
Thank you everyone for all your advice, I shall be having some good old position practice on Monday :) I'll do some no stirrup work as unfortunately my stirrups are at their longest already...
I think my hardest obstacle will be understand how to "hug" with the inside of my calf, yet not pinch at the knee... I'm struggling to see how I'm going to do this, but I'll try really hard!
I did attempt to keep my toes pointing forward during my lesson last night, but I may have been doing it wrong as my ankles were very very sore the whole lesson.
I used to take riding lessons with a girl who's toes always pointed out. It was just the way she was built and no amount of stretching, exercise or practice could change it. She eventually just had to realize that this is the way her body is and she rode like that. I'm not saying that this is your case but if you try everything that has been said and it is very painful to keep your toes forward it might be that you are just built that way.
When you first get on your horse and several times during the ride, place your hand under the back of your thigh and pull all the muscle of your thigh to the back. This will put your thigh flat and turn your knee and toe forward.
When you finish riding there should be no grease from the horse on the outside of the seam of your boots.