My apologies in advance for the absence of italics in my reply; shrinking text for the examples was out of the question! We’ve always used Hart’s Rules here (I’m very surprised no one has brought this up in all these years), and there’s no capitalization under their convention. Here’s the OUP link:

http://www.oxforddictionaries.com/words/colon

I understand some have cited the Chicago Manual of Style. As I understood it, the capitalization after a colon that it advises is to cover cases such as captions, e.g.:

Above: Some examples of text. Tinker believed these did not aid legibility. Below: A secondary example.

As far as I can recall, even in US typesetting, until the late ’90s, there was no capitalization after a colon in text. This began emerging around 1999, and since then, it has become commonplace Stateside. I’d be interested to learn if my recollection is correct.

I started as a proofreader in the 1980s, had stints on computer terminals on phototypesetting machines before most people had heard of the word font, and I now publish. I kept more than a casual eye on how these things developed, and even remember discussing it with a colleague in New York in 2001 when I worked there, a few years after the post-colon capital began appearing Stateside.

I’m not attacking those who feel there should be a capital; I just wanted to get an extra perspective into this long-running discussion.