In early drafts of this announcement I had written “there is a significant number of unofficial Hermes webmail login forms”, which a couple of my colleagues corrected to “there are”, so I checked. I had thought that my version was pedantic but correct, but I may have got that impression from an over-simplified version of the rule. It turns out (according to the Economist style guide and other sources) that the rule is “the number is” but “a number are”. Subtle.
The best guideline is to rephrase away from controversial matters of style, which is what I did :-)