
The original type are M3 thread size, black, 8mm long socket button head machine screws.
Replacement 8mm screws are a bit hard to get. But on most machines, 10mm long screws fit okay, but may be slightly tight as you finish screwing them in (as the screw cleans the last bit of the thread in the threaded hole).
You don't have to use the same type. I use M3, 10mm, black, Pozidriv Pan head screws

I have some of these available for 4 pence each plus the price of post & packing (cost of an envelope about 10 pence, a second class stamp is currently 50 pence), so the cost for a set of 6 (including P&P) is 84 pence.
Mark