I’m in the London Marathon, I have heel pain and I’m running in panic. Instructions please?

You say you struggled with injury all the way through your training but I’m not sure you mean you struggled specifically with the heel injury through the 20 mile run. If you have I imagine you will be able to struggle through it during the race. If you need to feel ready for the marathon, let me tell you, if you did the 20 miler you ARE ready. Now it is just a question of winding down so you are rested and your body recuperated from the hard work you have done. You will not lose your fitness. Running without pain will benefit you more than further training.
A normal taper for a marathon would be a 20 mile run 3 weeks before the race and then you cut down to 75%, 50% and 25% of your maximum weekly mileage the following 3 weeks. You might want to take a week off or of VERY light running (30 minutes every other day) and then 25% of your maximum weekly mileage to see if that resolves your heel problem (or see a doctor in case it is something that can be resolved with medical attention).
Good luck!