Thanks @dgrat from your post here I was able to work out that the fix to the problem is to change the SOCAT parameter RECVFROM to LISTEN, i.e. that fixed the problem I described directly:
socat UDP4-LISTEN:14550,fork UDP4-DATAGRAM:192.168.74.255:14550,broadcast,range=192.168.74.0/255
Working great now, this is a significant configuration people should seriously consider as standard. Anyone reading this should go read that post to install it properly, as a Linux service.