I am very suprised you got them to fit on tightly using normal bolts as the stud pattern is slightly different! Get some wobbly bolts to get them on secure as you may be causing a lot of pressure on your existing bolts which could cause a lovely snap
If they are 98 and you put one wheel bolt into the wheel on the car and look through where the other bolts go, each hole will be perfectly centred. With 100 you will notice that all three will be off centre.
Plus in most cases it'll say 98 or 100 on the back of the alloy wheel.
Finally I have recieved the alloys from abroad. They were descrbed as 4x100. As these alloys are multi fit on the rear side there is also 4x98 and other fitments do you think that these fit on the bravo?
Only one way to find out for certain, try and put one of them on the car, that won't take long and you'll soon see if you have any PCD or clearance issues