Thanks alot mate, very 'simply' put 
Do petrols hold their torque for long in the rev range than DERVs?
Petrol engines will tend to make their peak torque higher up the rev range.
Diesels tend to have peak torque across a wider % of the available revs.
Peak torque is sometimes described as 80+% of max torque. (it's the description i'm using here).
This is why DERV torque curves tend to look flat, and petrol torque curves tend to look like a hill.
This is obviously a bit of a sweeping statement, as you can get peaky diesels and petrols with flat torque curves.
When you start adding forced induction, you can get engines to do basically whatever you want.
Hope that answers your question without me being a car bore.
