I've read a few artivles that says you must boil extract regardless of what the instructions say. On reading up on John Bull Master Class products, they say "Ignore the people who say this, they do not know what they are talking about"
The product has been boiled before canning and to boil further would destroy the delecate balance of flavour and hops aroma.
I have had other kits that have had aroma hops added and to boil them would do the same thing.
It seems in this hobby it pays to listen to people and to learn but to brew how you want to.