I've been back and forth alot recently ( 6 times a year or more) and I must recommend Norwegian as the way to do it, for both price and convenience, if Gatwick is not a problem to you. You'll get something really cheap normally with Norwegian as long as you can book more than 6-8 weeks in advance, even if you do go on peak travel days, short notice is more variable. You do have to pay extra for bags and seats, but beyond that I find Norwegian to be excellent quality, excellent punctuality and they go into a proper airport at Arlanda, and not some **** end of nowhere airport!
Graham