Air 2000 operate flights from STN every Monday, think they still operate under a charter number although all the TFS flights are now under scheduled number.
I think you should be able to book seat only though. Their website is
www.air2000.com. They have just been awarded 2 Travel Weekly Globe Awards, Best UK Charter Airline and their tour operator First Choice have also been awarded Best All-inclusive Tour Operator. In addition to that, three months ago Air 2000 won the other prestigious Top Charter Airline Award at the TTG Travel Industry Awards.
So they should be good.