Depending on the choice of routes, April would be the most sensible start time. I presume this is why April 2003 was the initial target start date. Easter is a good time to get people travelling and it gives a good jump off platform for the summer.
However, NOW would have to manage the salary situation with the people who had already started.
I'm not sure the lease companies would wait that long either, but that would depend on whether NOW had moved past the LOI stage and paid the deposit. A bit of hard negotiation might get them to defer the lease start date.
The only other problem with waiting until April would be that Summer slots would have to be confirmed. I think the slot conference was last week, so hopefully yhey have secured any slots they need. Having said that, is Luton slot controlled??