I think there are three ATR42s left, and when the order for the ATR72-500s was placed a while ago, it was said that they are on the way out. So probably not a long-term alternative.
The other question is, of course, in what kind of deal Aer Arran obtained the slots at LCY. If the slots have come from the airport itself (rather than from another carrier selling them to Aer Arran) - wouldn't the airport require a new carrier to use an aircraft of a certain size? My understanding is that no new slots are allocated for anything smaller than 70 seats (of course an airline can always promise a lot and show up with a smaller aircraft when push comes to shove), but this might be an urban legend.