Occasionally when I brake, possibly a tad harder than a nice gentle stop, not talking wheel locking ABS working its socks off here, I get a sharp metallic cracking, knocking noise from the NSR as best I can tell on my Vrs.

I mention Vrs because of the multilink rear axle maybe having something to do with it?

Any ideas please?

could be the brake pad has broke off the metal backing plate.