Ok, very simply:
As an example; I have two tasks, both are 10 days but one is 100% complete and one is 0% complete. The first task took 10 men 10 days to complete, however the second task takes 1 man 10 days to complete. I should therefore have % complete greater than 50% but it doesn't - it says I am only 50% complete because it sees the duration is 20 days and ten days are finished = 50%. But the work to do the first task takes more effort than the second and in reality I should be 90% complete. So what is the problem, what is wrong? It's driving me nuts.

Try % Work complete added to the summary task line.