Dave,
Just to add some further information for you:
MAN: 3 or 4 x 757-200, 2 x 757-300, 1 x A320
LGW: 3 or 4 x 757-200, 2 x A320
BHX: 2 x 757-200
NCL: 2 x 757-200
GLA: 3 x 757-200
STN: 1 x A320
EMA: 1 x A320
The 2 A330s are shared between MAN and LGW.
These figures are for the summer, I know that both NCL and GLA lose an aircraft in the winter.
As for initial basing, there is no fixed pattern for assignment, it really is a case of where the vacancies are - this year there was lots of MAN and LGW needed, as well as a few at GLA and NCL. It is more likely that as a new joiner you will go on the 757. There is some movement between fleets but as wheelbarrow says this is usually onto the A330, but if the fleet changes then the airline will most likely require pilots to undertake conversion courses!!
Hope this helps.
Wheelbarrow, check your email please!