Sorry I forgot to mention that I have tried usual sources including screwfix and local builders merchants, 100mm are to long they would hit the brickwork (can i use any other screws, or do they cause problems when skimmed over)
I know this reply might be a bit late for the OP (I hope it is)
But I thought I would add this reply in case anyone else is in a similar situation & comes across this thread, just like I did.
Best screws I have managed to find are the Timberdeck 4.0 x 85mm decking screws from Screwfix.
I have just purchased a sample box & they seem ideal not much thicker that a proper drywall screw, I just hope the coating on them will be OK with the plaster.
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