Heap was born in 1954 in Hampshire, England. As a child she would write a draw to reflect her rootless lifestyle, living in such places as Singapore and Egypt. Later she earned an art college degree from the Hull College of Art .
Typical of Heap's illustrative style are people lacking necks with large gaps between their eyes.
Heap is well-known for her illustrations in Jacqueline Wilson's books, such as in How to Survive Summer Camp (ISBN 978-0192727046) and Double Act (ISBN 978-0440413745), co-illustrating the latter with Nick Sharratt.