I am due to move into a new house in a couple of weeks which has a
rotten front bay window which I will be looking to replace very soon.
I am wondering the cheapest way to replace this with double glazing and
was hopeing that someone may be able to give me some advice with the
following questions..

which tends to be cheaper, large national companies or local tradesmen?
is it better to measure and buy the wondows first then just get someone
in to fit them?
is it difficult to fit your own double glazing? and if so do you need
to meet any building regulations.

also if anyone has any idea approximatelly a bay window with the
following rough measurements would be to fit that would also be very
helpful:

large pane approx 1800 mm * 1500mm
2 * small panes 750 mm * 1500mm

thanks in advance for your help.