I would simply tell the lab what you have done and let them sort it for you .
Ive often used Kodak 400 film and forgot to alter the film speed setting from 200 or 100 which slightly over exposes your film and gives you marginaly slower shutter speeds but some times the results can be quite pleasing .
Im not altogether sure about flash photography though . :dunno:
Feel free to kick my ass if I got it wrong . :sweat: