The area could have been plyed over the platform before sheeting with 100 guage polythene. Ae the pipes coming up from underneath you (vertical) or through your work area horizontally? It would be safer to have got the licensed scaffolder to ranch out the whole of the area completely then split the total area into phases so asbestos can be safely removed with suitable air changes. It's more manageable but ften more expensive, particularly as you will have to have one 4 stage clearance for each phase (therefore dragging the analyst back each time). If the pipework runs horizontally its much easier cot a circular hole in the poly and tape around it to create a seal. I would always have a negative pressure minitor connected near the wall where the pipe leads through and have your third man checking this for leaks. Make sure it is smoke tested regularly.

The pipework is running vertically. The platform is close boarded to the pipework (with a 6" gap for removal of lagging), so initially there isnt a problem. The problem comes once we have removed the lagging (6") thus creating a 12" gap.

Noting that I know little about the actual job so here goes: I think that you have discovered your problem. All you now need to do is answer it and move on from there e.g. as holes are made that should be immediately covered - This is not a hard thing to do provided its planned in from the start e.g a proactive suitable and sufficient RA with S&S method statements forms a S&S work plan If this is an afterthought you may need to pay more to get these holes filled in as they are made