In reply to Rog Wilko:
A single chain link can also be pressed into service at a push, likewise forged rap-rings and as someone else has already mentioned, mid-size hexes. A chain link is the sort of 20pence thing you can just drop in your chalk bag and forget about (usually until long after the epic where you actually needed it!).
I never cease to be amazed by the number of people that plan for eventualities like dropping a belay plate, it's one of those times where you just make do.
I've dropped pretty much everything but my belay plate. Only time I've missed it (I occasionally forget it) was getting off Lost Arrow Spire in the rapidly descending evening gloom. Memorable for sure but not enough to make me even consider carrying a spare.
jk