Northflight has nothing to do with Signature who will likely relieve you of a large amount of cash for handling/parking. However, best plan would be to call them and ask (01224 723636) - I'm not aware of the current rates.
Getting in is no problem, the controllers do a great job of fitting in GA (such as it is there) around all the commercial ops, but do try to avoid departing when it's busy on the pax side otherwise you might be at the hold for 30/40 mins!
Bill's a great chap, I'm doing my IMC with him out of Aberdeen in the very near future.