please help - 5Dii and 550EX fill not working

i've been asked to take some photos at a baptism tomorrow but am having trouble getting the flash to work as fill flash and am getting severe overexposure.

i haven't used the flash for some time, so i'm not sure if i'm missing something obvious or if it's something else.

basically, i am shooting in Av mode.  at ISO 100 as far as i can tell everything works as expected.  however i will be shooting in a church tomorrow so need to increase ISO, but if i do this (say i set it to 800) then flash exposure increases too, resulting in massive overexposure of the shot.

another thing i noticed is that the distance meter reads around 9 metres, even though the object i'm focussing on is only a metre or two away.

one last thing is that FEC doesn't seem to have an effect.  even if i set FEC to -2 or -3 stops, the exposure looks exactly the same.

shouldn't the flash compensate automatically when i increase ISO?  and how come the distance meter is so far off?

could do with some help asap on this as i need to get it sorted by tomorrow morning.

thanks in advance.