It's pretty much how CX747 has said, the VS023 and VS017 are operated on A340's and the rest are on B744's, but sometimes the VS009 operates on A340.
They swap the destinations between the summer and winter periods for capacity, i.e. The 747's did the HKG/JNB routes over the winter (busy time for those routes) and the DEL/LOS routes were operated on the 742's until they went, now they are A340 routes, which means that we don't have as many A340's free to operate the transatlantic routes.
Basically if you want to see VS 744's departing go to LHR before 1430, if you want to see the A340's departing, go after 1430.
This will probably all change later in the year when we get the A346's. (end of July now I heard the othe day - due to further setbacks).
CX - I only thought we got one of our own (G-VWOW)....I'm probably wrong tho.....
"She Rolls, 45 knots, 90, 135, nose comes up to 20 degrees, she's airborne - She flies, Concorde Flies"