I am using WINSDMP from the NTRESKIT, this dumps out the wins records from a given Server, in csv format, and appears to give me what I need.

I just pull out all of the 0 type records (which I think is workstation service) this gives me a list of around 650 PCs in about 45 seconds.

Obviously Printers etc which would have responded to a ping may not be talking to WINS, but its only PCs that interest me.