Mikey21,
Generally for the 340-300 I use 7T/hr for the trip fuel. Then after adding on all the other bits:
Taxi
Contingency
Mandatory
Alternate
Reserve
...Which will vary, you come up with a figure about 12T more than the trip fuel.
So on a 12hr flight (London), expect something like 96T total fuel.
The 340-600 uses a bit more, something like 9T/hr. SO the 15hr flight to New York will have approx 147T total.
This weather being good, if alternates are needed then depending where they are it will bump the fuel up a bit higher.
I don't know about the 747 at all, maybe someone else can answer that one.