>For that reason, I believe that the key to opening up the truth of the purpose of henges lies in the function of the ditch.<

Think you've put your finger on it although, as FourWinds says -

>It's quite possible that the purpose of the ditch was to provide soil for the bank!<

In which case it's more the bank than the ditch that's the key.

So perhaps we're back to the question of the bank - ie <i>Which side of the ditch is the spoil from the ditch deposited?</i> If the spoil is on the <i>inside</i> of the ditch that would seem to indicate a defensive structure. If the spoil is on the outside of the ditch that would seem to indicate a sacred, ceremonial or utilitarian (or a combination of all three) use for the structure.

My bet's on the latter.