I really doubt Stobart have any plan for Aer Arann. It suited them to invest in them during their examinership a couple of years ago because it gave them a vehicle on which to launch their 'new look' Southend airport. Given that Southend now has several other operators using the airport including easyJet, it has served it's purpose.
I think Stobart increasing it's stake is more to do with it recovering it's investment than using it as part of some strategy to increase it's presence in the airline market. Stobart will be investing a lot of money that will be used to allow Aer Arann purchase new ATR 72 aircraft, and this comes with a lot of risk, and taking up their option to increase their stake makes sense as it provides them with greater security in the face of such risk while also allowing them to recoup their investment quicker.