What made you change the alternator? It is possible for a drain to occur via the alt, if the rectification side is bad, despite it being new.

Have you measured the power at the battery to see the charge rate etc?

If the switch has simply been replaced it won't be that as you haven't touched the wiring side (The live is before the switch) so it would have always been like that, unless of course someone has altered the wiring incorrectly, you would also hear the pump running and it would drain the power very quickly (Assuming it's Petrol)

Martin