Thanks for replying. I'm using FireFox and they have discontinued their error console! Used Chrome, same problem. Enabled define('SCRIPT_DEBUG', true); but it didn't report anything.
Found out that the two I tried to block whilst debug was enabled were actually blocked, it is just that the page didn't return and the spinner graphic just kept spinning.
This ONLY happens on the Top Consumers section. Blocking IPs in other sections returns the page.
Thanks