Ok, have been doing some more work on this...

Question:- When I'm downloading a large test file why do unrelated hops further down the chain sometimes show a greater increase in latency than those at the top where you would expect the load to manifest?

Possible answer:- Is it possible that in some circumstances servers can do the opposite of what we normaly expect and return icmp packets faster than they process normal traffic.? If so then this some through traffic latency might not show up at the earlier hops but would still be added to the later ones.???

Just a thought,
Ed.