Hi Pete
Have you asked for a copy of your telephone assessment report? If you haven't done so - then you should ring the DWP so you can understand how they came to their decision. What you should also do is look back at what you've written. Having a heart attack is a dreadful event and you say having four has changed your life forever. What you need to do is check that you have made it clear how it has changed in your ability to fulfill the specific PIP criteria on each question. You say they're not taking it seriously and that may be the case - but go through each question when you get the report and see if there's more you need to say.
BIS
Nothing on this board constitutes legal advice - always consult a professional about specific problems