neville,
why hasn't there been a study of the total radiation whilst flying
I'm not sure why you are under the impression that is the case. The issue of elevated levels of radiation with increasing altitudes was noted in the early 20th century and has been the subject of (physics) research ever since. The Physics journals contain many studies - even articles the dread wiki gives some indication of the amount of research that has been undertaken over the years.
Without getting too bogged down in either stats or folklore surrounding this whole issue I'm another who feels that radiation is way down the list of things that are injurious to our flying health ( thinks disrupted sleep, irregular exposure to daylight, airline meals
)