If a crew goes sick at Norwich running a A320 you have to reposition one of the spare 738s with full crew, not as easy as sending a taxi with just crew members or parts with an engineer who doesn't work on an Airbus routinely. I think one of the 320s ended up at Luton towards the end of the summer season last year because it was easiler to keep it going from there. Maybe they have learned and will keep any leased 320s at bigger bases where disruption can be managed better.